One pretends to be someone else (8)
I believe the answer is:
impostor
'one pretends to be someone else' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for impostor that I've seen before include "Sit or mop (anag.)" , "He fraudulently pretends to be someone else" , "Charlatan - is to romp (anag)" , "One assuming a false character" , "Person who pretends to be someone else" .)