Chaps an inexperienced driver notices (4)
I believe the answer is:
lads
'chaps' is the definition.
('lad' can be a synonym of 'chap')
'inexperienced driver notices' is the wordplay.
'inexperienced driver' becomes 'l' (as in L-plates for learner drivers).
'notices' becomes 'ads' (advertisements).
'l'+'ads'='LADS'
'an' acts as a link.
(Other definitions for lads that I've seen before include "Young men" , "Fellows, blokes" , "Fellows, chaps" , "Youngsters" , "Boys, young fellows" .)