Offence caused by frost on top of car (5)
I believe the answer is:
crime
'offence' is the definition.
(thesaurus)
'frost on top of car' is the wordplay.
'frost' becomes 'rime' ('rime' can be a synonym of 'frost').
'on' says to put letters next to each other.
'top of' indicates taking the first letters.
The first letter of 'car' is 'c'.
'rime' put after 'c' is 'CRIME'.
'caused by' is the link.
(Other definitions for crime that I've seen before include "Illegal activity" , "Speeding, perhaps" , "Violation of law" , "Illegal deed" , "Transgression of the law" .)