Loud noise made by horse and bird (4)
I believe the answer is:
howl
'loud noise' is the definition.
(thesaurus)
'horse and bird' is the wordplay.
'horse' becomes 'H' (abbreviation as in horse power).
'and' says to put letters next to each other.
'bird' becomes 'owl' (owl is a kind of bird).
'h'+'owl'='HOWL'
'made by' acts as a link.
(Other definitions for howl that I've seen before include "Long wailing cry" , "Long cry of pain" , "Long, loud, emotional utterance" , "Laugh (or cry?)" , "Long, loud, emotional cry" .)