Sort of coal to use when you've money to burn (7)
I believe the answer is:
lignite
'sort of coal' is the definition.
'money to burn' is the wordplay.
'money' becomes 'l' (L can mean 'pounds').
'to burn' becomes 'ignite' (igniting is a kind of burning).
'l'+'ignite'='LIGNITE'
'to use when you've' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for lignite that I've seen before include "rock" , "Brown coal" , "Soft brownish coal" , "Fuel" , "it'll burn" .)