Wedding vow line – a test for over-the-top devotion (8)
I believe the answer is:
idolatry
'devotion' is the definition.
(synonyms)
'wedding vow line a test' is the wordplay.
'wedding vow' becomes 'I do' ('I do' is part of the traditional wedding vows).
'line' becomes 'l' (used when specifying particular lines from a poem).
'test' becomes 'try' (synonyms).
'ido'+'l'+'a'+'try'='IDOLATRY'
'for over-the-top' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for idolatry that I've seen before include "Adulation" , "Getting worked up by image" , "Worship of a false god" , "Icon worship" , "Hero worship" .)