Bill catches large fish (5)
I believe the answer is:
bleak
'fish' is the definition.
(I've seen this before)
'bill catches large' is the wordplay.
'bill' becomes 'beak' (synonyms).
'catches' indicates putting letters inside.
'large' becomes 'L' (eg in clothes sizes).
'beak' placed around 'l' is 'BLEAK'.
(Other definitions for bleak that I've seen before include "raw > fish" , "Cold and desolate" , "Exposed and barren" , "offering little comfort" , "Austere" .)