The clearance as you have provided it is nonsensical. On a precision approach, the FAF is the glideslope intercept altitude, or an altitude that is lower when provided vectors to glideslope intercept at a lower altitude.
Accordingly, ATC cannot say maintain XXX until the FAF...because the FAA occurs at the published glide slope intercept altitude (GSIA). If you're assigned XXX altitude "prior to the FAF," and that altitude is higher than GSIA, you'll never reach the FAF because you'll have to descent to get there...FAF dosn't exist on the glideslope until passing through GSIA.
I believe I get the gist of your question, but the wording is still incorrect, and multiple answers apply depending on your wording.
Generally an apporoach clearance will contain an altitude to maintain until established. For example, one might be told "Three miles from KNOLE, maintain three thousand until established, cleared the Jahooliwooli ILS 23C." In this case, you maintain three thousand feet until you're established on the approach. One established, if lower published altitudes apply, such as step downfixes, you may follow them prior to glideslope intercept, or you may wait for the glidesope.
You do not have glideslope gaurantees above or beyond the published GSIA, and for you the glideslope is advisory-only until passing the GSIA...but you can intercept it and follow it down instead of stepping down with stepdown fixes...so long as you take care to ensure that you're meeting the altitude restrictions of each stepdown as you continue down the glideslope.
I watched an experienced captain in a light jet some years ago intercept a false glideslope and attempt to follow it down. I alerted him repeatedly to the fact that he was too far out and that his descent did not conform to the step down fixes; he was below minimum IFR altitudes for that approach segment. He felt that because he had crossed needles that he was okay, and he wasn't. False signals should occur above the glideslope, not below it where they did...but they were viable signals, the aircraft was coupled, and despite valid information showing him he was in the wrong place, he would have followed it into the ground.
Many pilots don't bother to verify that their descent is going according to plan; once the needles are crossed, they blindly follow the signal down, never verifying that they are indeed meeting their minimum altitude-distances at each fix they pass, on the way down. I'm very big on that, and outside the published FAF and before GSIA, I put each altitude in the alerter in succession as we pass each step down fix...even though we're coupled on the glideslope above GSIA.
I know a lot of folks don't do this, and many would consider it overkill, but I've seen what can happen when you don't. While not entirely the same, the GIII that was picking up former president Bush in November 2004 would have benifitted from this same practice. Both pilots of that aircraft were experienced pilots, one a former chief pilot for the company, and the other the current chief pilot. They flew into the ground.
Without pounding the soapbox too much, you maintain your assigned altitude until established. At that point, you may descend using the published stepdown fixes, or you may intercept the glidesope, if appropriate. If you do intercept the glideslope above GSIA, you're still responsible for ensuring that you meet each published minimum altitude until you're past the fix to which that altitude applies, during your descent on the glideslope. Doing so, while unfortunately not a common practice, ensures proceedure compliance, terrain and obstacle separation, and is acting in the best interests of safety...as well as ensures that your legal obligations in executing the approach are met with respect to altitude. It may just save your bacon.
