Don is one motorist missing start (5)
I believe the answer is:
river
'don is one' is the definition.
(I know that Don is a river)
'motorist missing start' is the wordplay.
'motorist' becomes 'driver' (I've seen this before).
'missing start' means to remove the first letter.
'driver' with its first letter taken away is 'RIVER'.
(Other definitions for river that I've seen before include "eg Aire" , "x or y, so to speak" , "Large natural stream of water" , "Eg, Severn" , "Eg, Amazon" .)