Piece of music composed without a key? (8)
I believe the answer is:
Here is my best explanation:
'piece of music' is the definition.
(serenade is a kind of piece of music)
'composed without a key?' is the subsidiary indication.
'composed' becomes 'serene' (I've seen this in other clues**).
'without' indicates putting letters inside.
'key?' becomes 'd' (I've seen this in other clues).
'serene' going around 'ad' is 'serenade'.
(Other definitions for serenade that I've seen before include "Romantic song under fair lady's window", "Evening music (played outside woman's house)", "Song of romance", "Piece sung beneath beloved's window", "Murder", "Song sung by suitor", "Musical performance, under a lady's window?", "Noted performance", "(Romantic?) song or tune" and "Song to a lover".)