One assuming a false identity (8)
I believe the answer is:
impostor
'one assuming a false identity' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for impostor that I've seen before include "One assuming a false identity (O, not E)" , "He fraudulently pretends to be someone else" , "Fake figure" , "phoney!" , "Charlatan - is to romp (anag)" .)