Custom to marry following ceremony (8)
I believe the answer is:
formally
'custom to' is the definition.
I can't judge whether this defines the answer.
'marry following ceremony' is the wordplay.
'marry' becomes 'ally' (I can't explain this - if you can you should believe this answer much more).
'following' is a charade indicator (letters next to each other).
'ceremony' becomes 'form' (form is a kind of ceremony).
'ally' put after 'form' is 'FORMALLY'.
Can you help me to learn more?
(Other definitions for formally that I've seen before include "In decorous or ceremonial manner" , "With official authorisation" , "in precise terms" , "Officially" , "following conventional rules" .)