A second bird behind (6)
I believe the answer is:
astern
'behind' is the definition.
(I've seen this before)
'a second bird' is the wordplay.
'a second' becomes 'as' (I've seen this in other clues).
'bird' becomes 'tern' (tern is a kind of bird**).
'as'+'tern'='ASTERN'
(Other definitions for astern that I've seen before include "Towards a ship's rear" , "Towards the rear of a ship" , "Backward" , "Towards the back (of a ship)" , "State of S central US" .)