Rejected in its entirety - it's for the birds (4)
I believe the answer is:
nest
'it's for the birds' is the definition.
(I've seen this before)
'rejected in its entirety' is the wordplay.
'rejected' shows that the letters should be reversed in order.
'in' says the answer is hidden in the clue.
'tsen' is hidden within 'its entirety'.
'tsen' back-to-front is 'NEST'.
(Other definitions for nest that I've seen before include "Home of twigs, perhaps" , "Rook's residence?" , "Cosy retreat" , "Sent to bird's home" , "Structure built by birds" .)