Sailor's behind a small bird (6)
I believe the answer is:
astern
'sailor's behind' is the definition.
'astern' can be an answer for 'behind' (at the back or behind of a ship). I'm not certain of the 'sailor's' bit.
'a small bird' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'bird' becomes 'tern' (tern is a kind of bird).
'a'+'s'+'tern'='ASTERN'
Can you help me to learn more?
(Other definitions for astern that I've seen before include "Towards the rear of boat" , "Nautically behind" , "Ran set (anag.)" , "Towards the rear of a ship" , "Towards back of boat" .)