Get to know when positive (9)

I believe the answer is:

ascertain

'get to know' is the definition.
(ascertaining is a kind of getting to know)

'when positive' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'positive' becomes 'certain' (similar in meaning).
'as'+'certain'='ASCERTAIN'

(Other definitions for ascertain that I've seen before include "Get to know" , "Learn" , "settle" , "Find out for sure, determine" , "Find out - make sure" .)

I've seen this clue in The Telegraph.

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