Jacket, etc, that motorist's in most often? (3,4)
I believe the answer is:
top gear
'that motorist's in most often?' is the definition.
(as in driving in top gear)
I can't explain the remainder of the clue.
(Other definitions for top gear that I've seen before include "Fashion" , "one may drive in it" , "series on TV" , "Perhaps fourth" , "5th in a car?" .)