Get to know when sure (9)
I believe the answer is:
ascertain
'get to know' is the definition.
(ascertaining is a kind of getting to know)
'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 "Prove" , "Establish after an investigation or study" , "Confirm, make sure" , "Get to know" , "Establish, find out" .)