It's grand as fuel (3)
I believe the answer is:
gas
'fuel' is the definition.
(gas is a kind of fuel)
'it's grand as' is the wordplay.
'it's grand' becomes 'g' ('G' can be a synonym of 'grand'**).
'g'+'as'='GAS'
(Other definitions for gas that I've seen before include "Eg, nitrogen" , "Eg, hydrogen" , "Domestic fuel" , "Something delightful" , "Carbon dioxide, say" .)