With perfect exactness, carbon copies captured a collection of notes (7)
I believe the answer is:
toccata
'collection of notes' is the definition.
(toccata is a kind of piece of music)
'with perfect exactness carbon copies captured a' is the wordplay.
'with perfect exactness' becomes 'to a T'.
'carbon copies' becomes 'cc' (common abbreviation in correspondence).
'captured' means one lot of letters goes inside another.
'toat' enclosing 'cc' is 'toccat'.
'toccat'+'a'='TOCCATA'
(Other definitions for toccata that I've seen before include "Noted work" , "Touching piece" , "Keyboard piece emphasising touch" , "Musical work designed to show player's technique" , "Musical composition for keyboard to show technique" .)