Less than average gent using public transport upset an unexperienced driver (9)

'an unexperienced driver' is the 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?

'less than average gent using public transport upset' is the wordplay.
I cannot quite understand how this works, but
'average' could be 'norm' (synonyms) and 'norm' is located in the answer.
'gent' could be 'al' and 'al' is found in the answer.
'transport' could be 'sub' (sub is a kind of transport) and 'sub' is found within the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?