Scorch with hot top in vehicle (4)
I believe the answer is:
char
'scorch' is the definition.
(synonyms)
'hot top in vehicle' is the wordplay.
'top' suggests taking the first letters (in a down clue, the first letter is top).
'in' indicates putting letters inside.
'vehicle' becomes 'car' (car is a kind of vehicle).
The first letter of 'hot' is 'h'.
'h' put within 'car' is 'CHAR'.
'with' is the link.
(Other definitions for char that I've seen before include "Partly burn - cleaning woman" , "Brown" , "Clean < fish" , "Burn; scorch" , "Partially burn; fish" .)