Reg and Maria involved in the union (8)
I believe the answer is:
marriage
'the union' is the definition.
('marriage' can be a synonym of 'union')
'reg and maria involved' is the wordplay.
'and' is a charade indicator (letters next to each other).
'involved' indicates anagramming the letters (involved can mean confusing or complex).
'reg'+'maria'='regmaria'
'regmaria' with letters rearranged gives 'MARRIAGE'.
'in' is the link.
(Other definitions for marriage that I've seen before include "nuptials" , "The nuptial ceremony" , ""Till death do us part" arrangement" , "'The - - of Figaro'" , "Matrimony" .)