Small barrier due to be bought (9)
I believe the answer is:
swallowed
'bought' is the definition.
(swallowing is a kind of buying)
'small barrier due' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'barrier' becomes 'wall' (a wall acts as a barrier).
'due' becomes 'owed' (synonyms).
's'+'wall'+'owed'='SWALLOWED'
'to be' acts as a link.
(Other definitions for swallowed that I've seen before include "Meekly accepted" , "Ingested" , "Gulped" , "believed" , "drunk maybe" .)