Closed union may be set up to employ such a model (4)
I believe the answer is:
nude
'such a model' is the definition.
(as in a nude model)
'closed union may be set up to employ' is the wordplay.
'to employ' says the answer is hidden in the clue (the answer's letters are employed by the clue).
'may be set up' shows that the letters should be reversed in order.
'edun' is hidden within 'closed union'.
'edun' backwards is 'NUDE'.
(Other definitions for nude that I've seen before include "Naked figure" , "Completely unclothed" , "Without cover" , "Appearing as nature intended?" , "Unclad" .)