Gas from terribly pert old student (6)
I believe the answer is:
petrol
'gas' is the definition.
(I know that gas can be written as petrol)
'terribly pert old student' is the wordplay.
'terribly' indicates anagramming the letters.
'old' becomes 'o' (abbreviation).
'student' becomes 'L' (as in L-plates for learner drivers).
'pert' with letters rearranged gives 'petr'.
'petr'+'o'+'l'='PETROL'
'from' is the link.
(Other definitions for petrol that I've seen before include "The juice" , "Liquid fuel" , "Fuel for road vehicles" , "Gasoline" , "Mineral oil refined as fuel" .)