Small whale boat (4)
I believe the answer is:
scow
'boat' is the definition.
(scow is a kind of boat)
'small whale' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'whale' becomes 'cow' (I can't explain this - if you can you should give a lot more credence to this answer).
's'+'cow'='SCOW'
Can you help me to learn more?
(Other definitions for scow that I've seen before include "Unpowered freight barge" , "Low boat" , "ighter" , "Flat-bottomed boat for cows" , "Maybe lighter" .)