Confirm when definite (9)

I believe the answer is:

ascertain

'confirm' is the definition.
(ascertaining is a kind of confirming)

'when definite' is the wordplay.
'when' becomes 'as'.
'definite' becomes 'certain' ('certain' can be similar in meaning to 'definite').
'as'+'certain'='ASCERTAIN'

(Other definitions for ascertain that I've seen before include "Confirm, make sure" , "Cartesian (anag)" , "Prove" , "Find out for sure, determine" , "Establish (the facts)" .)

I've seen this clue in The Telegraph and The Globe and Mail.

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