Duke, working men's benefactor (5)
I believe the answer is:
donor
'benefactor' is the definition.
(donor is a kind of benefactor)
'duke working men's' is the wordplay.
'duke' becomes 'd'.
'working' becomes 'on'.
'men' becomes 'or' (military abbreviation for Other Ranks).
'd'+'on'+'or'='DONOR'
(Other definitions for donor that I've seen before include "Philanthropist" , "One who gives, maybe blood" , "A giver, often of blood" , "patron" , "One who gives something" .)