Set fire to brown coal left unused (6)
I believe the answer is:
ignite
'set fire' is the definition.
(to ignite something is to set it on fire)
'brown coal left unused' is the wordplay.
'brown coal' becomes 'lignite' (synonyms).
'left' becomes 'L' (common abbreviation).
'unused' indicates named letters should be taken away.
'lignite' with 'l' taken away is 'IGNITE'.
'to' acts as a link.
(Other definitions for ignite that I've seen before include "Touch off; inflame" , "Detonate" , "Torch" , "Cause to catch fire" , "Set fire to, light up" .)