Terrifying force once affected stoppage with power cut (7)
I believe the answer is:
gestapo
'terrifying force' is the definition.
'Gestapo' can be an answer for 'force' (Nazi secret police). I'm not certain of the 'terrifying' bit.
'affected stoppage with power cut' is the wordplay.
I cannot quite understand how this works, but
'stoppage' could be 'tap' (tap is a kind of stoppage) and 'tap' is present in the answer.
This explanation may well be incorrect...
'once' acts as a link.
Can you help me to learn more?
(Other definitions for gestapo that I've seen before include "Notorious secret police" , "Nazi Germany's secret state police" , "Geheime Staatspolizei" , "war force" , "Men in black" .)