A small seabird at the back of the ship (6)
I believe the answer is:
astern
'at the back of the ship' is the definition.
'a small seabird' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'seabird' becomes 'tern' (tern is a kind of seabird).
'a'+'s'+'tern'='ASTERN'
(Other definitions for astern that I've seen before include "Behind at sea" , "Nautically behind" , "(In relation to ship) behind" , "The wake is" , "Behind (a boat)" .)