A second bird at the back of the ship (6)
I believe the answer is:
astern
'at the back of the ship' is the definition.
'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 "Toward the tail of a vessel" , "following behind vessel" , "State of S central US" , "Behind (a boat)" , "Rearward" .)