Fleet in surprise attack seizing power (5)
I believe the answer is:
rapid
'fleet' is the definition.
(I've seen this before)
'surprise attack seizing power' is the wordplay.
'surprise attack' becomes 'raid' (I've seen this before).
'seizing' means one lot of letters goes inside another.
'power' becomes 'p'.
'raid' going around 'p' is 'RAPID'.
'in' is the link.
(Other definitions for rapid that I've seen before include "Speedy, quick" , "Express" , "Precipitate" , "Waterway where the current runs very fast" , "Speedy (5)" .)