Ring friend conserving energy (4)
I believe the answer is:
peal
'ring' is the definition.
('peal' can be a synonym of 'ring')
'friend conserving energy' is the wordplay.
'friend' becomes 'pal' (pal is a kind of friend).
'conserving' means one lot of letters goes inside another.
'energy' becomes 'e' (physics symbol as in E=mc2).
'pal' going around 'e' is 'PEAL'.
(Other definitions for peal that I've seen before include "Deep prolonged sound" , "Ring recurrently" , "Carillon" , "Thunderous roar" , "Leap about to sound of bells" .)