Advert interrupting peaceful piece of music (8)
I believe the answer is:
serenade
'piece of music' is the definition.
(serenade is a kind of piece of music)
'advert interrupting peaceful' is the wordplay.
'advert' becomes 'ad' (synonyms).
'interrupting' means one lot of letters goes inside another.
'peaceful' becomes 'serene' (similar in meaning).
'ad' inserted into 'serene' is 'SERENADE'.
(Other definitions for serenade that I've seen before include "Sing (for someone on a balcony?)" , "Romantic song under fair lady's window" , "Sing to (lover)" , "Open-air song" , "Piece of music for lovers at night" .)