Catalogue includes unknown composer (5)
I believe the answer is:
liszt
'composer' is the definition.
(Franz Liszt)
'catalogue includes unknown' is the wordplay.
'catalogue' becomes 'list' (a catalogue is a list of items).
'includes' means one lot of letters goes inside another.
'unknown' becomes 'z' (algebra).
'list' going around 'z' is 'LISZT'.
(Other definitions for liszt that I've seen before include "music writer" , "Hungarian piano virtuoso, d. 1886" , "Pianist, father of Cosima Wagner" , "Franz . . . . ., great Hungarian composer" , "Hungarian composer/pianist" .)