Knowing one needs to start class of French on the way back (8)
I believe the answer is:
informed
'knowing' is the definition.
('informed' can be similar in meaning to 'knowing')
'one needs to start class of french on the way back' is the subsidiary indication.
'one' becomes 'i' (roman numeral).
'to start' indicates taking the first letters.
'class' becomes 'form' (form class is a kind of class**).
'of french' becomes 'de'.
'on the way back' says the letters should be written backwards.
The initial letter of 'needs' is 'n'.
'de' back-to-front is 'ed'.
'i'+'n'+'form'+'ed'='informed'
(Other definitions for informed that I've seen before include "Told" , "Imparted knowledge" , "Expert" , "Gave knowledge to" , "Passed on knowledge" .)