Most wary of learner driver on scary road (8)
I believe the answer is:
leeriest
'most wary' is the definition.
(I've seen this in another clue)
'learner driver on scary road' is the wordplay.
'learner driver' becomes 'l' (learner drivers have L-plates on their cars).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'scary' becomes 'eerie' (I've seen this in another clue).
'road' becomes 'st'.
'l'+'eerie'+'st'='LEERIEST'
'of' acts as a link.
(Other definitions for leeriest that I've seen before include "Most guarded" , "Most careful" .)