Fat fish (4)
I believe the answer is:
chub
'fat fish' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for chub that I've seen before include "Plump fish" , "Small river fish of the carp family" , "Fat relative of the carp" , "Fat freshwater fish" , "River fish; rhymes with public house" .)