Great obscurity about a member of two unions? (8)
I believe the answer is:
bigamist
'member of two unions?' is the definition.
(I've seen this before)
'great obscurity about a' is the wordplay.
'great' becomes 'big' (similar in meaning).
'obscurity' becomes 'mist'.
'about' says to put letters next to each other.
'mist' put after 'a' is 'amist'.
'big'+'amist'='BIGAMIST'
(Other definitions for bigamist that I've seen before include "member of two unions" , "Person who marries illegally - already wed (8)" , "One with two other halves?" , "One who unlawfully marries a second wife" , "One married to two people at once" .)