Fleet shows power in attack (5)
I believe the answer is:
rapid
'fleet' is the definition.
(I've seen this before)
'power in attack' is the wordplay.
'power' becomes 'p'.
'in' means one lot of letters goes inside another.
'attack' becomes 'raid' (raiding is a kind of attacking).
'p' inserted within 'raid' is 'RAPID'.
'shows' acts as a link.
(Other definitions for rapid that I've seen before include "Express" , "Quick or swift" , "Fast, quick" , "Flying" , "Expeditious" .)