Start to drive with adulterated petrol — a small quantity (7)
I believe the answer is:
droplet
'quantity' is the definition.
(droplet is a kind of quantity)
'start to drive with adulterated petrol a small' is the wordplay.
I cannot really understand how this works, but
'drive' could be 'dr' (I've seen this in other clues) and 'dr' is present in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for droplet that I've seen before include "Small tear of fluid" , "Small quantity of liquid" , "tiny amount of liquid" , "Small amount of liquid" , "Tiny bead of liquid" .)