Butt, or small cask (4)
I believe the answer is:
stub
'butt' is the definition.
(synonyms)
'small cask' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'cask' becomes 'tub' (I've seen this before).
's'+'tub'='STUB'
'or' is the link.
(Other definitions for stub that I've seen before include "Butt of pencil or cigarette" , "Counterfoil, of a cheque say" , "30 [FAG END]" , "Wee bit of pencil, eg," , "Tooth remnant" .)