Now, interrupting the calm, comes music (8)
I believe the answer is:
serenade
'music' is the definition.
(serenade is a kind of music)
'now interrupting the calm' is the wordplay.
'now' becomes 'AD' (AD, Anno Domini, as opposed to BC, Before Christ).
'interrupting' is an insertion indicator.
'the calm' becomes 'serene' (I've seen this before).
'ad' going into 'serene' is 'SERENADE'.
'comes' is the link.
(Other definitions for serenade that I've seen before include "Play music, sing for someone" , "Murder" , "(Romantic?) song or tune" , "Evening music (played outside woman's house)" , "Piece of music for lovers at night" .)