Casino in Ulster partly backed by workers' group (5)
I believe the answer is:
union
'workers' group' is the definition.
(I've seen this in another clue)
'casino in ulster partly backed' is the wordplay.
'partly' indicates the answer is hidden within the clue (part of the clue is used).
'backed' says the letters should be written backwards.
'noinu' is hidden within 'casino in ulster'.
'noinu' written backwards gives 'UNION'.
'by' is the link.
(Other definitions for union that I've seen before include "workers' co-operative" , "Marriage; pipe joint" , "Marriage or alliance" , "Wedding, say" , "Joining into one - association" .)