Animal in river in Derby? (4)
I believe the answer is:
hart
'animal' is the definition.
(I've seen this before)
'river in derby?' is the wordplay.
'river' becomes 'r'.
'in' indicates putting letters inside.
'derby?' becomes 'hat' (derby is a kind of hat).
'r' inserted within 'hat' is 'HART'.
'in' is the link.
(Other definitions for hart that I've seen before include "providing material for Richard Rodgers?" , "Beast" , "Male deer; Rodgers collaborator" , "Richard Rodgers and Lorenz . . . . were famous musical partnership" , "Deer; Lorenz --, lyricist" .)