Be discriminatory, but not at first (5)
I believe the answer is:
exist
'be' is the definition.
(synonyms)
'discriminatory but not at first' is the wordplay.
'discriminatory' becomes 'sexist' (similar in meaning).
'but not at first' means to remove the first letter.
'sexist' with its initial letter removed is 'EXIST'.
(Other definitions for exist that I've seen before include "quick!" , "Survive" , "Subsist" , "Have being or continue living" , "Get a life!" .)