Good when used for cooking (3)
I believe the answer is:
gas
'for cooking' is the definition.
Although both the answer and 'cooking' are verbs, they are not in the same form of the verb.
'good when' is the wordplay.
'good' becomes 'g' (abbreviation).
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'g'+'as'='GAS'
'used' acts as a link.
Can you help me to learn more?
(Other definitions for gas that I've seen before include "It's not liquid or solid, like American petrol" , "Something delightful" , "Fumes, vapour" , "Hydrogen or methane, say" , "Something that is neither solid not liquid" .)