First-class fuel for stoves (4)
I believe the answer is:
agas
'stoves' is the definition.
(I've seen this before)
'first-class fuel' is the wordplay.
'first class' becomes 'A' (as in A-list, A-team etc).
'fuel' becomes 'gas' (gas is a fuel).
'a'+'gas'='AGAS'
'for' acts as a link.
(Other definitions for agas that I've seen before include "Stoves" , "kitchen appliances" , "cookers and heaters" , "Appliances used for cooking" , "commanders" .)