Bird following a second behind (6)
I believe the answer is:
astern
'behind' is the definition.
(I've seen this before)
'bird following a second' is the wordplay.
'bird' becomes 'tern' (tern is a kind of bird**).
'following' says to put letters next to each other.
'a second' becomes 'as' (I've seen this in other clues).
'tern' after 'as' is 'ASTERN'.
(Other definitions for astern that I've seen before include "Ran set (anag.)" , "Toward the back of a boat" , "(In relation to ship) behind" , "Behind at sea" , "Behind (naut.)" .)