Cosmetic: not the best for much longer? (9,5)
I believe the answer is:
vanishing cream
'cosmetic' is the definition.
(I've seen this before)
'not the best for much longer?' is the wordplay.
I cannot really understand how this works, but
'best' could be 'cream' (as in 'the cream of the crop') and 'cream' is found within the answer.
The remaining letters 'vanishing' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for vanishing cream that I've seen before include "foundation?" , "Brain Drain" , "Cosmetic" .)