Attack in uncertain tones (5)
I believe the answer is:
onset
'attack' is the definition.
(archaic term for a military attack)
'uncertain tones' is the wordplay.
'uncertain' indicates an anagram.
'tones' is an anagram of 'ONSET'.
'in' acts as a link.
(Other definitions for onset that I've seen before include "Kick-off" , "Beginning (of disease etc)" , "Appearance of" , "Beginning; military attack" , "Storming" .)