Weakling has fight to cut into fish (6)
I believe the answer is:
coward
'weakling' is the definition.
(thesaurus)
'fight to cut into fish' is the wordplay.
'fight' becomes 'war' (warring is a kind of fighting).
'to cut into' means one lot of letters goes inside another.
'fish' becomes 'cod' (type of fish).
'war' put into 'cod' is 'COWARD'.
'has' acts as a link.
(Other definitions for coward that I've seen before include "Person who is deficient in courage" , "Gutless type" , "Chicken" , "Theatrical fellow" , "Yellow-belly" .)