Bird returning in autumn, a lot roosting (7)
I believe the answer is:
ortolan
'bird' is the definition.
(ortolan is a kind of bird)
'returning in autumn a lot roosting' is the wordplay.
'returning' is a reversal indicator.
'in' says the answer is hidden in the clue.
'nalotro' is hidden within 'autumn a lot roosting'.
'nalotro' reversed gives 'ORTOLAN'.
(Other definitions for ortolan that I've seen before include "One flying" , "Bird" , "Bobolink" , "Small songbird" , "high-flying singer" .)