Energy source, or an energy metaphor (3)
I believe the answer is:
gas
I believe this clue is a double definition.
'energy source' is the first definition.
(I've seen this in another clue)
'an energy metaphor' is the second definition.
(I've seen this in another clue)
(Other definitions for gas that I've seen before include "Chat (colloq.)" , "Fumes, vapour" , "Something exciting" , "idle chit-chat" , "Chatter vapidly" .)