Learn when determined (9)
I believe the answer is:
'learn' is the definition.
(I know that learn can be written as ascertain)
'when determined' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'determined' becomes 'certain' (certain can mean determined or decided).
(Other definitions for ascertain that I've seen before include "Learn" , "Become aware" , "Find out by making enquiries in sectarian way" , "Establish after an investigation or study" , "Find out for sure, determine" .)