Quiet, skilled workman, one devoted to a cause (8)
I believe the answer is:
partisan
'one devoted to a cause' is the definition.
(supporter or fighter for a particular cause)
'quiet skilled workman' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'skilled workman' becomes 'artisan' (I've seen this before).
'p'+'artisan'='PARTISAN'
(Other definitions for partisan that I've seen before include "Resistance fighter" , "Inclining to one side" , "In Sparta (anag)" , "Prejudiced towards one side" , "Aspirant to be a guerilla" .)