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' means one lot of letters go next to another.
'energy' becomes 'e' (physics symbol as in E=mcl).
'cuts' becomes 'clips' (clipping is a kind of cutting).
The initial letter of 'expected' is 'e'.
'e'+'clips'='eclips'
'e' put 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" .)