Fool pursues river bird (6)
I believe the answer is:
falcon
'bird' is the definition.
(falcon is a kind of bird)
'fool pursues river' is the wordplay.
'fool' becomes 'con' (I've seen this in another clue).
'pursues' means one lot of letters go next to another.
'river' becomes 'fal' (River Fal).
'con' put after 'fal' is 'FALCON'.
(Other definitions for falcon that I've seen before include "Hunter winged" , "See 16" , "Clan of (anag) - raptor" , "Kind of hunting bird - peregrine is one" , "A hunting bird, may be peregine" .)