Period of darkness expected at first after energy cuts (7)
I believe the answer is:
eclipse
'period of darkness' is the definition.
(I've seen this before)
'expected at first after energy cuts' is the wordplay.
'at first' says to take the initial letters.
'after' is a charade indicator (letters next to each other).
'energy' becomes 'e' (physics symbol as in E=mcl).
'cuts' becomes 'clips' (clipping is a kind of cutting).
The first letter of 'expected' is 'e'.
'e'+'clips'='eclips'
'e' after 'eclips' is 'ECLIPSE'.
(Other definitions for eclipse that I've seen before include "Celestial phenomenon" , "screen, more or less" , "Obscuring of a heavenly body" , "Momentary disappearance of heavenly body" , "Solar/lunar phenomenon" .)