Marriage plan with suit ordered (8)
I believe the answer is:
nuptials
'marriage' is the definition.
(thesaurus)
'plan with suit ordered' is the wordplay.
'with' says to put letters next to each other.
'ordered' indicates an anagram.
'plan' put after 'suit' is 'suitplan'.
'suitplan' anagrammed gives 'NUPTIALS'.
(Other definitions for nuptials that I've seen before include "A wedding or marriage" , "Wedding ceremony" , "Marriage ceremonies" , "Wedding - suit plan (anag)" , "I slant up for a wedding" .)