Set fire to fuel left out (6)
I believe the answer is:
ignite
'set fire' is the definition.
'fuel left out' is the wordplay.
'fuel' becomes 'lignite' (type of coal).
'left' becomes 'l' (common abbreviation).
'out' indicates named letters should be taken away.
'lignite' with 'l' taken out is 'IGNITE'.
'to' acts as a link.
(Other definitions for ignite that I've seen before include "Cause to catch fire" , "Kindle Fire" , "Burn" , "Combust" , "Swan constellation" .)