Register includes unknown Hungarian composer (5)
I believe the answer is:
liszt
'hungarian composer' is the definition.
(I've seen this before)
'register includes unknown' is the wordplay.
'register' becomes 'list' (listing is a kind of registering).
'includes' is an insertion indicator.
'unknown' becomes 'z' (algebra).
'list' placed around 'z' is 'LISZT'.
(Other definitions for liszt that I've seen before include "Pianist, father of Cosima Wagner" , "Franz . . . . ., great Hungarian composer" , "Brahms and - is rhyming slang for drunk" , "He scored" , "Hungarian composer/pianist" .)