Complain when dropping fifty in defeat (4)
I believe the answer is:
beat
'defeat' is the definition.
(I know that beat is a more specific form of the action defeat)
'complain when dropping fifty' is the wordplay.
'complain' becomes 'bleat' (bleating is a kind of complaining).
'when dropping' suggests deleting specific letters.
'fifty' becomes 'l' (Roman numeral).
'bleat' with 'l' removed is 'BEAT'.
'in' is the link.
(Other definitions for beat that I've seen before include "Thrash, punish physically" , "Policeman's round" , "Get the better of in competition" , "Regular route of a policeman" , "tired" .)