Learner set fire to fuel (7)
I believe the answer is:
lignite
'to fuel' is the definition.
(type of coal)
'learner set fire' is the wordplay.
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'set fire' becomes 'ignite' (to ignite something is to set it on fire).
'l'+'ignite'='LIGNITE'
(Other definitions for lignite that I've seen before include "Brown coal" , "Fuel" , "Soft brownish coal" , "rock" , "it'll burn" .)