Notorious libertine composed sad, sad requiem (7,2,4)
I believe the answer is:
marquis de sade
'notorious libertine' is the definition.
(I know that Marquis de Sade is notorious)
'composed sad sad requiem' is the wordplay.
'composed' indicates anagramming the letters.
'sad'+'sad'+'requiem'='sadsadrequiem'
'sadsadrequiem' anagrammed gives 'MARQUIS DE SADE'.
(Other definitions for marquis de sade that I've seen before include "French libertine, d. 1814" , "Cruel and debauched French writer and soldier" .)