Gas from terribly pert old student (6)
I believe the answer is:
petrol
'gas' is the definition.
(thesaurus)
'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'.
'petr'+'o'+'l'='PETROL'
'from' acts as a 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" .)