When, without doubt, you’ll discover (9)
I believe the answer is:
ascertain
'discover' is the definition.
(I know that ascertain is a more specific form of the action discover)
'when without doubt' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'without doubt' becomes 'certain' (I am not sure about this - if you are sure you should believe this answer much more).
'as'+'certain'='ASCERTAIN'
'you'll' acts as a link.
I am not very happy about this link. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for ascertain that I've seen before include "Learn" , "Find out by making enquiries in sectarian way" , "Establish after an investigation or study" , "Find out - make sure" , "Become aware" .)