Gas from terribly pert old student (6)
I believe the answer is:
Here is my best explanation:
'gas' is the definition.
'terribly pert old student' is the wordplay.
'terribly' indicates anagramming the letters.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'student' becomes 'l' (as in L-plates for learner drivers).
'pert' with letters rearranged gives 'petr'.
'from' acts as a link.
(Other definitions for petrol that I've seen before include "Car fuel", "Gasoline", "Fuel for road vehicles", "Mineral oil refined as fuel", "Common car fuel", "Liquid fuel", "Common motor fuel", "The juice", "Motor fuel" and "Fuel for the internal combustion engine".)