Bird and small horse lying around in pool next to river (9)
I believe the answer is:
merganser
'bird' is the definition.
(merganser is a kind of bird)
'small horse lying around in pool next to river' is the wordplay.
'small horse' becomes 'nag' (I've seen this before).
'lying around' says the letters should be written backwards.
'in' is an insertion indicator.
'pool next' becomes 'merse' (I can't justify this - if you can you should believe this answer much more).
'to river' becomes 'r' (abbreviation for river).
'nag' reversed gives 'gan'.
'gan' going within 'merse' is 'merganse'.
'merganse'+'r'='MERGANSER'
'and' is the link.
(Other definitions for merganser that I've seen before include "Diving duck" , "Diving bird" , "Diver" , "Sawbill" , "Together saw bill" .)