Don is one motorist missing start (5)
I believe the answer is:
river
'don is one' is the definition.
(I know this)
'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 initial letter taken off 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" .)