German composer, hard one, working the mind (9)
I believe the answer is:
hindemith
'german composer' is the definition.
(composer Paul Hindemith)
'hard one working the mind' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'one' becomes 'I' (Roman numeral).
'working' is an anagram indicator.
'the'+'mind'='themind'
'themind' with letters rearranged gives 'ndemith'.
'h'+'i'+'ndemith'='HINDEMITH'
(Other definitions for hindemith that I've seen before include "Composer" , "for his music" .)