Quick to attack, besieging quietly (5)
I believe the answer is:
rapid
'quick' is the definition.
(synonyms)
'attack besieging quietly' is the wordplay.
'attack' becomes 'raid' (raid is a kind of attack).
'besieging' indicates putting letters inside.
'quietly' becomes 'p' (musical symbol).
'raid' going around 'p' is 'RAPID'.
'to' is the link.
(Other definitions for rapid that I've seen before include "Zippy" , "Waterway where the current runs very fast" , "Electrical connector" , "Moving at speed" , "Flying" .)