Crust of snow at front on vehicle (4)
I believe the answer is:
scab
'crust' is the definition.
(I've seen this in another clue)
'snow at front on vehicle' is the wordplay.
'at front' indicates taking the first letters.
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'vehicle' becomes 'cab' (cab is a kind of vehicle).
The initial letter of 'snow' is 's'.
's'+'cab'='SCAB'
'of' is the link.
(Other definitions for scab that I've seen before include "Crust over sore or strike-breaker" , "Strike breaker" , "Disease of sheep" , "Dried crust on sore" , "Wound's crust" .)