Swimmer in river, then bus avoiding cold (5)
I believe the answer is:
roach
'swimmer' is the definition.
(roach is a kind of fish)
'river then bus avoiding cold' is the wordplay.
'river' becomes 'r'.
'then' is a charade indicator (letters next to each other).
'bus' becomes 'coach' ('coach' can be a synonym of 'bus').
'avoiding' is a deletion indicator.
'cold' becomes 'c' (eg on taps).
'coach' with 'c' removed is 'oach'.
'r'+'oach'='ROACH'
'in' is the link.
(Other definitions for roach that I've seen before include "Fish - part of a cannabis cigarette" , "Small freshwater fish related to carp" , "Freshwatwer fish" , "member of family carp" , "Silvery freshwater fish" .)