Grievance of one in factory (6)
I believe the answer is:
plaint
'grievance' is the definition.
(I have seen 'Legal grievance' mean 'plaint' so perhaps 'grievance' could also mean 'plaint')
'one in factory' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' means one lot of letters goes inside another.
'factory' becomes 'plant' (a manufacturing plant).
'i' placed into 'plant' is 'PLAINT'.
'of' acts as a link.
(Other definitions for plaint that I've seen before include "Archaic word for grievance (6)" , "Legal grievance" , "Lamentation or grievance" , "Legal suit" , "Mournful song" .)