Small bird's back (5)
I believe the answer is:
stern
'back' is the definition.
(stern means the back of a boat)
'small bird's' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'bird' becomes 'tern' (tern is a kind of bird).
's'+'tern'='STERN'
(Other definitions for stern that I've seen before include "Severe and unremitting: area of a ship" , "Harsh at the back of the ship" , "Severe -- back of boat" , "Back end of a ship" , "Strict - forbidding" .)