Offering resistance that's set in stone (12)

I believe the answer is:

oppositional

'offering resistance' is the definition.
(providing opposition)

'set in stone' is the wordplay.
'set' becomes 'position' (to set is to position or locate).
'in' is an insertion indicator.
'stone' becomes 'opal' (I've seen this before).
'position' put within 'opal' is 'OPPOSITIONAL'.

'that's' is the link.

I've seen this clue in The Times.

Want a hint initially instead of a full solution? Install my app