Like a bird at the back (6)
I believe the answer is:
'at the back' is the definition.
(I've seen this before)
'like a bird' is the wordplay.
'like' becomes 'as'.
'a bird' becomes 'tern' (tern is a kind of bird).
(Other definitions for astern that I've seen before include "Nautically behind" , "Toward the back of a boat" , "back for sailors" , "Toward the tail of a vessel" , "Behind the back of a ship" .)