Bird nightly entertains returning primate (5)
I believe the answer is:
indri
'primate' is the definition.
(indri is a kind of primate)
'bird nightly entertains returning' is the wordplay.
'entertains' indicates the answer is hidden within the clue (the answer is welcomed or entertained by the clue).
'returning' shows that the letters should be reversed in order.
'irdni' can be found hidden inside 'bird nightly'.
'irdni' backwards is 'INDRI'.
(Other definitions for indri that I've seen before include "Species of lemur" , "native of Madagascar" , "Animal" , "large primate" , "Madagascan mammal" .)