Fish in a butt (5)
I believe the answer is:
roach
This clue is a double definition.
'fish' is the first definition.
(roach is a kind of fish)
'a butt' is the second definition.
(thesaurus)
'in' acts as a link.
(Other definitions for roach that I've seen before include "Fish of the carp family" , "member of family carp" , "Something fishy here" , "Silvery freshwater fish" , "Fresh-water food fish" .)