Quick! Quietly into attack! (5)
I believe the answer is:
rapid
'quick' is the definition.
(synonyms)
'quietly into attack' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'into' is an insertion indicator.
'attack' becomes 'raid' (raid is a kind of attack).
'p' put inside 'raid' is 'RAPID'.
(Other definitions for rapid that I've seen before include "Precipitate" , "Waterway where the current runs very fast" , "Speedy, quick" , "Fast, quick" , "Brisk" .)