Fine performer sitting on unknown works (7)
I believe the answer is:
factory
'works' is the definition.
(factory is a kind of works)
'fine performer sitting on unknown' is the wordplay.
'fine' becomes 'f' (abbreviation).
'performer' becomes 'actor' (actor is a kind of performer).
'sitting on' is a charade indicator (letters next to each other).
'unknown' becomes 'y' (algebra).
'f'+'actor'+'y'='FACTORY'
(Other definitions for factory that I've seen before include "Building used for making or assembling items" , "Place for production" , "Works" , "Manufacturing plant" , "mill" .)