Piece of music composed without a key? (8)
I believe the answer is:
serenade
'piece of music' is the definition.
(serenade is a kind of piece of music)
'composed without a key?' is the wordplay.
'composed' becomes 'serene' (I've seen this before).
'without' indicates putting letters inside ('without' can be similar in meaning to 'outside').
'key?' becomes 'd' (I've seen this in other clues**).
'a'+'d'='ad'
'serene' enclosing '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 of music for lovers at night" , "Murder" .)