Use the gas for nothing but pudding (4)
I believe the answer is:
sago
'pudding' is the definition.
(I've seen this before)
'use the gas for nothing' is the wordplay.
'use' indicates anagramming the letters.
'for' is a charade indicator (letters next to each other).
'nothing' becomes 'O' (looks like zero - 0).
'gas' anagrammed gives 'sag'.
'sag'+'o'='SAGO'
'but' is the link.
(Other definitions for sago that I've seen before include "Cereal, used in pudding" , "Edible palm starch" , "dessert?" , "Powdery starchy food" , "Starchy foodstuff" .)