Parking on road the wrong way is punishable in law (5)
I believe the answer is:
penal
'punishable in law' is the definition.
(penal means relating to punishment)
'parking on road the wrong way' is the wordplay.
'parking' becomes 'p' (abbreviation on road signs).
'on' is a charade indicator (letters next to each other).
'road' becomes 'lane' (a lane is a small road).
'the wrong way' shows that the letters should be reversed in order.
'lane' backwards is 'enal'.
'p'+'enal'='PENAL'
'is' acts as a link.
(Other definitions for penal that I've seen before include "Punishing" , "Corrective" , "Involving punishment as by the law" , "Involving legal punishment" , "Deliberately unpleasant" .)