Manner of one in workforce (4)
I believe the answer is:
mien
'manner' is the definition.
(mien can mean a manner or attitude)
'one in workforce' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' means one lot of letters goes inside another.
'workforce' becomes 'men'.
'i' placed within 'men' is 'MIEN'.
'of' acts as a link.
(Other definitions for mien that I've seen before include "(One's) manner, bearing" , "Facial expression sounds miserly" , "The carriage" , "A countenance like mine" , "A manner like mine" .)