Barrage of questions Sally let go, keeping quiet (5,4)
I believe the answer is:
rapid fire
'barrage of questions' is the definition.
(as in rapid-fire questioning)
'sally let go keeping quiet' is the wordplay.
'sally' becomes 'raid' (both can mean a military attack).
'let go' becomes 'fire' (both can mean to dismiss from a job).
'keeping' means one lot of letters goes inside another.
'quiet' becomes 'p' (music symbol for 'piano').
'raid'+'fire'='raidfire'
'raidfire' going around 'p' is 'RAPID-FIRE'.
(Other definitions for rapid fire that I've seen before include "Going off quickly" , "Like a barrage of questions" , "Asking of questions in quick succession" .)