Loving marriage vow is written in loveless language (9)
I believe the answer is:
idolising
'loving marriage vow is written in loveless language' is the definition.
I can't judge whether this definition defines the answer.
'vow is written in loveless language' is the wordplay.
'vow' becomes 'I do' (I do - vow said at weddings).
'written in' means one lot of letters goes inside another.
'loveless language' becomes 'ling' (I can't explain this - if you can you should believe this answer much more).
'is' going within 'ling' is 'lising'.
'ido'+'lising'='IDOLISING'
Can you help me to learn more?
(Another definition for idolising that I've seen is " Hero-worshipping".)