Two notes interrupting calm musical piece (8)
I believe the answer is:
serenade
'musical piece' is the definition.
(I've seen this in another clue)
'two notes interrupting calm' is the wordplay.
'two notes' can mean two replacements for 'note'.
'note' becomes 'a' (musical note).
'note' becomes 'D' (musical note).
'interrupting' indicates putting letters inside.
'calm' becomes 'serene' (synonyms).
'a'+'d'='ad'
'ad' put into 'serene' is 'SERENADE'.
(Other definitions for serenade that I've seen before include "piece of romantic music" , "Open-air song" , "Music played by lover" , "Song to a lover" , "Murder" .)