Songwriter's catalogue includes riches, half unknown before (8)
I believe the answer is:
lyricist
'songwriter's' is the definition.
(I've seen this before)
'catalogue includes riches half unknown before' is the wordplay.
'catalogue' becomes 'list' (I've seen this before**).
'includes' is an insertion indicator.
'half' indicates taking half.
'unknown' becomes 'y' (algebra).
'before' means one lot of letters go next to another.
'riches' halved is 'ric'.
'ric' put after 'y' is 'yric'.
'list' enclosing 'yric' is 'LYRICIST'.
(Other definitions for lyricist that I've seen before include "Writer of words, as opposed to music" , "Book writer" , "Writer of words for songs" , "Sit Cyril down - he's a songwriter" , "Person who writes words for songs" .)