Horse in river finds carp (6)
I believe the answer is:
'carp' is the definition.
'horse in river' is the wordplay.
'horse' becomes 'gg'.
'in' indicates putting letters inside.
'river' becomes 'nile' (Nile is an example**).
'gg' going into 'nile' is 'NIGGLE'.
'finds' acts as a link.
(Other definitions for niggle that I've seen before include "Objection" , "Get" , "Persistently irritate" , "'Small, gnawing worry (6)'" , "Cause slight annoyance" .)