Exact lookalike (4)
I believe the answer is:
twin
'exact lookalike' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for twin that I've seen before include "Identical sibling" , "One of an identical pair" , "One of two that match" , "(Town) paired with another" , "One of two duplicates" .)