Workers' organisation (5)
'workers' organisation' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for union that I've seen before include "Marriage; pipe joint" , "Coupling, merger" , "Association - guild" , "(Pipe) coupling" , "Wedding, say" .)
(Other definitions for trade union that I've seen before include "Equity is one such" , "Equity perhaps" , "Organised Labour" .)