Darkness resulting from cuts in essentially indispensable energy (7)
I believe the answer is:
eclipse
'darkness' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'cuts in essentially indispensable energy' is the wordplay.
'cuts' becomes 'clips' (clip is a kind of cut).
'in' indicates putting letters inside.
'essentially' means to look at the middle letters.
'energy' becomes 'e' (physics symbol as in E=mcl).
The central letter of 'indispensable' is 'e'.
'e'+'e'='ee'
'clips' inserted inside 'ee' is 'ECLIPSE'.
'resulting from' acts as a link.
Can you help me to learn more?
(Other definitions for eclipse that I've seen before include "Hiding of one body by another" , "Decline" , "period of darkness" , "A deliquium" , "Obscuring of a heavenly body" .)