Quietly consume fuel (4)
I believe the answer is:
peat
'fuel' is the definition.
(peat is a kind of fuel)
'quietly consume' is the wordplay.
'quietly' becomes 'p' (musical symbol).
'consume' becomes 'eat' (eating is a kind of consuming).
'p'+'eat'='PEAT'
(Other definitions for peat that I've seen before include "Fuel or compost (but no longer to be used as such!)" , "Dried turf" , "Turf as in bogland" , "Partially carbonised vegetable matter saturated with water" , "Vegetable deposit used as fuel and fertiliser" .)