Smile crossing river to find swimmer (5)
I believe the answer is:
bream
'swimmer' is the definition.
(bream is a kind of aquatic vertebrate)
'smile crossing river' is the wordplay.
'smile' becomes 'beam' (beaming is a kind of smiling).
'crossing' indicates putting letters inside.
'river' becomes 'r'.
'beam' going around 'r' is 'BREAM'.
'to find' is the link.
(Other definitions for bream that I've seen before include "Carp relative" , "Fish - English guitarist" , "Broad, thin fish" , "River fish" , "Amber kind of fish" .)