Composer that’s unknown’s turning up in reading material (5)
I believe the answer is:
liszt
'composer' is the definition.
(Franz Liszt)
'unknown's turning up in reading material' is the wordplay.
'unknown's' becomes 'z's' (algebra).
'turning up' shows that the letters should be reversed in order.
'in' indicates putting letters inside.
'reading material' becomes 'lit' (short for literature).
'zs' back-to-front is 'sz'.
'sz' going within 'lit' is 'LISZT'.
'that's' acts as a link.
(Other definitions for liszt that I've seen before include "classical composer" , "Hungarian piano virtuoso, d. 1886" , "Hungarian romantic composer" , "half-drunk?" , "Hungarian composer/pianist" .)