Open to bribery, one is held pardonable (6)
I believe the answer is:
venial
'pardonable' is the definition.
(similar in meaning)
'open to bribery one is held' is the wordplay.
'open to bribery' becomes 'venal' (I've seen this before).
'one' becomes 'I' (Roman numeral).
'is held' means one lot of letters goes inside another.
'venal' enclosing 'i' is 'VENIAL'.
(Other definitions for venial that I've seen before include "Of sin, pardonable, not too serious" , "Forgivable (sin)" , "A not-too-serious sin" , "Not serious, like small sin" , "Pardonable (offence)" .)