Like bird’s behind (6)
I believe the answer is:
astern
'behind' is the definition.
(I've seen this before)
'like bird's' is the wordplay.
'like' becomes 'as'.
'bird' becomes 'tern' (tern is a kind of bird).
'as'+'tern'='ASTERN'
(Other definitions for astern that I've seen before include "Aft" , "Behind (naut.)" , "Rearward" , "Towards rear of a vessel" , "Towards the rear of a ship" .)