Minor annoyance for horse in river (6)
I believe the answer is:
niggle
'minor annoyance' is the definition.
(I've seen this before)
'horse in river' is the wordplay.
'horse' becomes 'gg'.
'in' means one lot of letters goes inside another.
'river' becomes 'nile' (Nile is an example**).
'gg' going into 'nile' is 'NIGGLE'.
'for' acts as a link.
(Other definitions for niggle that I've seen before include "Cause minor persistent annoyance" , "A slight bother" , "Persistant annoyance" , "Annoy slightly" , "Minor criticism - slight worry" .)