Quick attack seizing power (5)
I believe the answer is:
rapid
'quick' is the definition.
(synonyms)
'attack seizing power' is the wordplay.
'attack' becomes 'raid' (raid is a kind of attack).
'seizing' means one lot of letters goes inside another (inserted letters are seized).
'power' becomes 'p' (symbol in physics).
'raid' placed around 'p' is 'RAPID'.
(Other definitions for rapid that I've seen before include "Precipitate" , "Waterway where the current runs very fast" , "Brisk" , "Happening at great speed" , "Prompt" .)