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