Extensive lack of clarity about a union fraudster (8)
I believe the answer is:
bigamist
'union fraudster' is the definition.
(person fraudulent about marriages ie unions)
'extensive lack of clarity about a' is the wordplay.
'extensive' becomes 'big' (similar in meaning).
'lack of clarity' becomes 'mist' (mist can mean vagueness or lack of clarity).
'about' means one lot of letters go next to another.
'mist' put after 'a' is 'amist'.
'big'+'amist'='BIGAMIST'
(Other definitions for bigamist that I've seen before include "One with a spouse too many" , "Person who marries while still married to another" , "member of two unions" , "One who has two spouses at the same time" , "Marital cheat" .)