A small bird showing behind (6)
I believe the answer is:
astern
'behind' is the definition.
(I've seen this before)
'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'
'showing' acts as a link.
(Other definitions for astern that I've seen before include "following behind vessel" , "Behind the back of a ship" , "The wake is" , "Abaft" , "Ran set (anag.)" .)