Find out when? Sure (9)

I believe the answer is:

ascertain

'find out' is the definition.
(ascertaining is a kind of finding out)

'when? sure' is the wordplay.
'when?' becomes 'as'.
'sure' becomes 'certain' ('certain' can be a synonym of 'sure').
'as'+'certain'='ASCERTAIN'

(Other definitions for ascertain that I've seen before include "settle" , "Find out - make sure" , "Discover" , "Become aware" , "Prove" .)

I've seen this clue in The Telegraph and The Guardian.

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