Find out when confident (9)
I believe the answer is:
ascertain
'find out' is the definition.
(synonyms)
'when confident' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'confident' becomes 'certain' (similar in meaning).
'as'+'certain'='ASCERTAIN'
(Other definitions for ascertain that I've seen before include "Establish (the facts)" , "Cartesian (anag)" , "settle" , "Verify" , "Discover" .)