Tell a British Conservative it's instructive (11)

I believe the answer is:

informatory

'it's instructive' is the definition.
(associated in meaning)

'tell a british conservative' is the wordplay.
'tell' becomes 'in form' (I've seen this before).
'british conservative' becomes 'tory' (I've seen this in another clue).
'inform'+'a'+'tory'='INFORMATORY'

I've seen this clue in The Irish Independent.

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