Fail — as may loose dentures? (4,7)
I believe the answer is:
come unstuck
'fail' is the definition.
(I've seen this before)
I cannot understand how the remainder of the clue works.
(Other definitions for come unstuck that I've seen before include "Fail" , "experience a setback?" , "Suffer failure" , "detached?" , "Lose one's grip" .)