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' means one lot of letters goes inside another.
'large' becomes 'L' (eg in clothes sizes).
'beak' going 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" .)