Cut kills fish (7)
I believe the answer is:
topside
'cut' is the definition.
(I've seen this before)
'kills fish' is the wordplay.
'kills' becomes 'tops' ().
'fish' becomes 'ide' (type of fish).
'tops'+'ide'='TOPSIDE'
(Other definitions for topside that I've seen before include "Joint" , "Dish" , "a cut above the others?" , "Lean cut of beef, no bone" , "Outer cut of beef" .)