He'll drive a vehicle past (5)
I believe the answer is:
'he'll drive a' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'vehicle past' is the wordplay.
'vehicle' becomes 'cab' (cab is a kind of vehicle).
'past' becomes 'by' (as in years gone by).
Can you help me to learn more?
(Other definitions for cabby that I've seen before include "Taxi driver" , "Taxi-driver" .)