Socialist keeps fighting for payment (6)
I believe the answer is:
reward
'payment' is the definition.
(reward is a kind of payment)
'socialist keeps fighting' is the wordplay.
'socialist' becomes 'red' (red is a kind of socialist).
'keeps' means one lot of letters goes inside another.
'fighting' becomes 'war'.
'red' going around 'war' is 'REWARD'.
'for' is the link.
(Other definitions for reward that I've seen before include "Prize payment" , "Deserved prize" , "Recompense for worthy act" , "Payment for services rendered" , "Pay" .)