Record broken by unknown composer (5)
I believe the answer is:
liszt
'composer' is the definition.
(Franz Liszt)
'record broken by unknown' is the wordplay.
'record' becomes 'list' (I've seen this before).
'broken by' 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 "Famous composer" , "Pianist, father of Cosima Wagner" , "with his partner, drunk?" , "music writer" , "Franz -, Hung. pianist" .)