Fight with cross fish (6)
I believe the answer is:
tangle
'fight with' is the definition.
(thesaurus)
'cross fish' is the wordplay.
'cross' becomes 'T' (shape).
'fish' becomes 'angle' (angle can mean to fish).
't'+'angle'='TANGLE'
(Other definitions for tangle that I've seen before include "Snarl or embroil" , "Snarl-up" , "Confused knot of something" , "Mix-up" , "Trap < tall guy from Perth" .)