Scorer's firm marks a problem (8)
I believe the answer is:
composer
'scorer's' is the definition.
(thesaurus)
'firm marks a problem' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'marks' becomes 'm' (former German currency).
'a problem' becomes 'poser' (poser is a kind of problem).
'co'+'m'+'poser'='COMPOSER'
(Other definitions for composer that I've seen before include "Writer of music" , "Beethoven or Mozart, say" , "one to make notes" , "Eg, Arne, Bach, Chopin" , "Mozart, e.g." .)