Those below will, for a period, not be in operation (8)
I believe the answer is:
downtime
'in operation' is the definition.
The definition suggests a singular noun which matches the answer.
'those below will for a period not' is the wordplay.
I cannot quite see how this works, but
'below' could be 'down' (in a down clue, some letters go below others) and 'down' is located in the answer.
'period' could be 'time' (period of time) and 'time' is present in the answer.
No letters remain.
This explanation may well be incorrect...
'be' acts as a link.
Can you help me to learn more?
(Other definitions for downtime that I've seen before include "Period when computer is out of action" , "Unproductive hours" , "while doing nothing" .)