Nonsense about hire car one’s kept driving (10)
I believe the answer is:
torrential
'driving' is the definition.
(I've seen this before)
'nonsense about hire car one's kept' is the wordplay.
'nonsense' becomes 'rot' (rot can mean nonsense or rubbish).
'about' says the letters should be written in reverse.
'hire car' becomes 'rental' (I can't justify this - if you can you should give a lot more credence to this answer).
'one' becomes 'I' (Roman numeral).
'kept' means one lot of letters goes inside another.
'rot' written backwards gives 'tor'.
'rental' enclosing 'i' is 'rential'.
'tor'+'rential'='TORRENTIAL'
Can you help me to learn more?
(Other definitions for torrential that I've seen before include "Like heavy fall" , "(Of rain) very heavy" , "Flowing overwhelmingly" , "driving" , "Pelting" .)