Legitimate banning learner driver that’s very bad (5)
I believe the answer is:
awful
'very bad' is the definition.
(I've seen this before)
'legitimate banning learner driver' is the wordplay.
'legitimate' becomes 'lawful' ('lawful' can be a synonym of 'legitimate').
'banning' indicates named letters should be taken away.
'learner driver' becomes 'l' (I've seen this in other clues).
'lawful' with 'l' taken out is 'AWFUL'.
'that's' is the link.
(Other definitions for awful that I've seen before include "Really terrible, deplorable" , "Dreadful - spine-chilling" , "Dreadful and dire" , "shocking" , "dire result" .)