Fish with large abnormal growth thrown back (5)
I believe the answer is:
trawl
'fish' is the definition.
(trawling is a kind of fishing)
'with large abnormal growth thrown back' is the wordplay.
'with' says to put letters next to each other.
'large' becomes 'l'.
'abnormal growth' becomes 'wart'.
'thrown back' shows that the letters should be reversed in order.
'l'+'wart' is 'lwart'.
'lwart' back-to-front is 'TRAWL'.
(Other definitions for trawl that I've seen before include "Fishermen may use it" , "Net for fishing at sea" , "Drag a large net" , "Go fishing" , "Fish using a towed net" .)