Party, and not for a benefactor (5)
I believe the answer is:
donor
'for a benefactor' is the definition.
(donor is a kind of benefactor)
'party and not' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'and not' becomes 'nor' ('nor' means 'and also not').
'do'+'nor'='DONOR'
(Other definitions for donor that I've seen before include "Generous type" , "One who gives something" , "Philanthropist" , "One who gives, perhaps blood" , "One who gives, maybe blood" .)