Quick assault seizing power (5)
I believe the answer is:
rapid
'quick' is the definition.
(synonyms)
'assault seizing power' is the wordplay.
'assault' becomes 'raid' (I've seen this in another clue).
'seizing' means one lot of letters goes inside another (inserted letters are seized).
'power' becomes 'p' (symbol in physics).
'raid' enclosing 'p' is 'RAPID'.
(Other definitions for rapid that I've seen before include "Speedy (5)" , "Precipitate" , "Quick or swift" , "Expeditious" , "Prompt" .)