Small animal in boat (4)
I believe the answer is:
scow
'boat' is the definition.
(I know that scow is a type of boat)
'small animal' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'animal' becomes 'cow' (cow is a kind of animal).
's'+'cow'='SCOW'
'in' acts as a link.
(Other definitions for scow that I've seen before include "Maybe lighter" , "Wide-beamed dinghy" , "ighter" , "Vessel" , "Unpowered freight barge" .)