Plug ends up saving energy (6)
I believe the answer is:
spigot
'plug' is the definition.
(spigot is a kind of plug)
'ends up saving energy' is the wordplay.
'ends up' becomes 'spit' (I can't explain this - if you can you should give a lot more credence to this answer).
'saving' indicates putting letters inside.
'energy' becomes 'go' (I've seen this in other clues).
'spit' placed around 'go' is 'SPIGOT'.
Can you help me to learn more?
(Other definitions for spigot that I've seen before include "It's a water tap in America" , "Vent hole peg - US tap" , "Tap, faucet" , "Small peg or plug" , "Tap for barrel" .)