Small beast is lighter (4)
I believe the answer is:
scow
'is lighter' is the definition.
(I've seen this before)
'small beast' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'beast' becomes 'cow' (cow is a kind of beast**).
's'+'cow'='SCOW'
(Other definitions for scow that I've seen before include "Vessel" , "ighter" , "Maybe lighter" , "Wide-beamed dinghy" , "Low boat" .)