Parking in beginning in front of garage— it's an offence (8)
I believe the answer is:
speeding
'it's an offence' is the definition.
(speeding is a traffic offence)
'parking in beginning in front of garage' is the wordplay.
'parking' becomes 'p' (abbreviation on road signs).
'in' is an insertion indicator.
'beginning' becomes 'seed' (seed is a kind of beginning).
'front of' indicates taking the first letters.
The initial letter of 'garage' is 'g'.
'p' inserted into 'seed' is 'speed'.
'speed'+'in'+'g'='SPEEDING'
(Other definitions for speeding that I've seen before include "bowling" , "Driving too fast" , "Moving very fast" , "Travelling at an illegal rate" , "Travelling over the limit" .)