Certain to restrain organised workers in a stitch-up (6)
I believe the answer is:
suture
'a stitch-up' is the definition.
(a suture is a medical stitch)
'certain to restrain organised workers' is the wordplay.
'certain' becomes 'sure' (synonyms).
'to restrain' indicates putting letters inside.
'organised workers' becomes 'TU' (abbreviation for trade union).
'sure' enclosing 'tu' is 'SUTURE'.
'in' acts as a link.
(Other definitions for suture that I've seen before include "Surgical stitching" , "Material for sewing a wound" , "Medical stitch" , "Surgery stitch" , "Surgical thread, seam" .)