Small seabird at the back (5)
I believe the answer is:
stern
'back' is the definition.
(stern means the back of a boat)
'small seabird at the' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'seabird' becomes 'ern' (I've seen this in another clue).
'at' says to put letters next to each other (I've seen this in other clues).
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'ern' after 't' is 'tern'.
's'+'tern'='STERN'
(Other definitions for stern that I've seen before include "at the back of the galley!" , "Grimly serious; back" , "Rigid" , "Rear end" , "Unsmiling" .)