Great learner driver changing gear (5)
I believe the answer is:
large
'great' is the definition.
('large' can be similar in meaning to 'great')
'learner driver changing gear' is the wordplay.
'learner driver' becomes 'l' (I've seen this in other clues**).
'changing' is an anagram indicator.
'gear' with letters rearranged gives 'arge'.
'l'+'arge'='LARGE'
(Other definitions for large that I've seen before include "Generous" , "Bulky" , "One has liberty to be at it" , "broad" , "Of considerable size or extent" .)