Its driver may well have the knowledge (3)
I believe the answer is:
cab
'knowledge' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for cab that I've seen before include "It has the authority to confiscate the proceeds of crime, in short" , "Transport" , "Driver's compartment, in lorry say" , "Driving compartment" , "A taxi in New York" .)