Cab --- or its driver who got a bad tip (7)
I believe the answer is:
growler
This clue could be a double definition.
'cab' is the first definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'driver who got a bad tip' is the second definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'or its' acts as a link.
Can you help me to learn more?
(Other definitions for growler that I've seen before include "Dog perhaps" , "Perhaps bear" , "Iceberg" , "Unfriendly dog" .)