Fish in river caught by William (5)
I believe the answer is:
brill
'fish' is the definition.
(brill is a kind of fish)
'river caught by william' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'caught by' means one lot of letters goes inside another.
'william' becomes 'bill'.
'r' placed into 'bill' is 'BRILL'.
'in' acts as a link.
(Other definitions for brill that I've seen before include "Fab -- fish" , "Great" , "catch, perhaps" , "Champion" , "Flatfish; excellent" .)