Wary learner gets stuck into jumbo after six (8)
I believe the answer is:
vigilant
'wary' is the definition.
('vigilant' can be a synonym of 'wary')
'learner gets stuck into jumbo after six' is the wordplay.
'learner' becomes 'l'.
'gets stuck into' indicates putting letters inside.
'jumbo' becomes 'giant' ('giant' can be a synonym of 'jumbo').
'after' is a charade indicator (letters next to each other).
'six' becomes 'vi' (roman numeral).
'l' going within 'giant' is 'gilant'.
'gilant' put after 'vi' is 'VIGILANT'.
(Other definitions for vigilant that I've seen before include "Be watchful while I gilt van" , "On the lookout" , "Ever watchful" , "Keeping careful watch" , "Carefully observant or attentive" .)