Chattered, quietly unnerved (8)
I believe the answer is:
prattled
'chattered' is the definition.
('prattle' can be a synonym of 'chatter')
'quietly unnerved' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'unnerved' becomes 'rattled' (I've seen this before).
'p'+'rattled'='PRATTLED'
(Other definitions for prattled that I've seen before include "Chattered" , "Wittered on" .)