Act on denial by Republican benefactor (5)
I believe the answer is:
donor
'benefactor' is the definition.
(donor is a kind of benefactor)
'act on denial by republican' is the wordplay.
'act on' becomes 'do'.
'denial' becomes 'no' (no is a kind of denial).
'by' means one lot of letters go next to another.
'republican' becomes 'r' (abbreviation).
'do'+'no'+'r'='DONOR'
(Other definitions for donor that I've seen before include "One who gives, maybe blood" , "One giving something" , "One who gives, perhaps blood" , "One contributing" , "One who gives something" .)