What's good in the role of fuel (3)
I believe the answer is:
gas
'fuel' is the definition.
(gas is a kind of fuel)
'what's good in the role of' is the wordplay.
'what's good' becomes 'g' (abbreviation).
'in the role of' becomes 'as' (as in an actor appearing as a character).
'g'+'as'='GAS'
(Other definitions for gas that I've seen before include "Eg, nitrogen" , "Neon, for example" , "Gossip" , "See 2" , "that may quickly be stepped on" .)