One unable to speak about quiet attribute (6)
I believe the answer is:
impute
'attribute' is the definition.
('impute' can be a synonym of 'attribute')
'one unable to speak about quiet' is the wordplay.
'one' becomes 'i' (Roman numeral).
'unable to speak' becomes 'mute' (I've seen this before).
'about' indicates putting letters inside.
'quiet' becomes 'p' (music symbol for 'piano').
'mute' placed around 'p' is 'mpute'.
'i'+'mpute'='IMPUTE'
(Other definitions for impute that I've seen before include "Attribute (something bad) to someone" , "Uptime (anag.)" , "Attribute or ascribe - say fault" , "take into account" , "reckon" .)