Socialist has fix on how fight can be ended (5)
I believe the answer is:
'socialist' is the definition.
(pinko can mean someone with leftwing views)
'fix on how fight can be ended' is the wordplay.
'fix' becomes 'pin' (to pin is to fix in place).
'on' says to put letters next to each other.
'how fight can be ended' becomes 'KO' (knock out in boxing).
'has' acts as a link.
(Another definition for pinko that I've seen is " Left supporter's".)