Power grab initiated after defence perimeter broke up (9)
I believe the answer is:
decoupled
'power grab' is the definition.
The definition suggests a verb in its base form which matches the answer.
'initiated after defence perimeter broke up' is the wordplay.
I cannot really understand how this works, but
'up' is within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for decoupled that I've seen before include "Having broken loose" , "Unlinked" .)