Fleet quietly stopping attack (5)
I believe the answer is:
rapid
'fleet' is the definition.
(I've seen this before)
'quietly stopping attack' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'stopping' means one lot of letters goes inside another (stopping can mean blocking a gap).
'attack' becomes 'raid' (raiding is a kind of attacking).
'p' inserted inside 'raid' is 'RAPID'.
(Other definitions for rapid that I've seen before include "Electrical connector" , "Zippy" , "Speedy (5)" , "Speedy, quick" , "Brisk" .)