Organising Sue's opal wedding (8)
I believe the answer is:
espousal
'wedding' is the definition.
(espousal is a kind of wedding)
'organising sue's opal' is the wordplay.
'organising' indicates an anagram.
'sues'+'opal'='suesopal'
'suesopal' anagrammed gives 'ESPOUSAL'.
(Other definitions for espousal that I've seen before include "Acceptance with approval" , "championship" , "Backing, support" , "Adoption, support (of)" , "Action of adopting (a cause)" .)