Piece of music to start to calm jazz fan with touch of anxiety (7)
I believe the answer is:
toccata
'piece of music' is the definition.
(toccata is a kind of piece of music)
'to start to calm jazz fan with touch of anxiety' is the wordplay.
'start to' indicates taking the first letters.
'jazz fan' becomes 'cat' (slang term for a jazz fan).
'with' says to put letters next to each other.
'touch of' says to take the initial letters.
The first letter of 'calm' is 'c'.
The first letter of 'anxiety' is 'a'.
'to'+'c'+'cat'+'a'='TOCCATA'
(Other definitions for toccata that I've seen before include "Musical work designed to show player's technique" , "Flashy piece of music" , "High-speed keyboard piece" , "Baroque musical piece" , "Keyboard composition emphasising technique" .)