Sanctimonious little girl in car plant (10)
I believe the answer is:
aspidistra
'plant' is the definition.
(aspidistra is a kind of plant)
'sanctimonious little girl in car' is the wordplay.
I cannot quite see how this works, but
'sanctimonious' could be 'pi' (old-fashioned term for pious) and 'pi' is present in the answer.
'girl' could be 'dis' (I've seen this in other clues) and 'dis' is present in the answer.
'car' could be 'astra' (I've seen this before) and 'astra' is found within the remaining letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for aspidistra that I've seen before include "The biggest - (in the world) (Gracie Fields)" , "Houseplant of lily family" , "Large houseplant" , "For Asian native" , "Plant of the lily family" .)