Wedding vow: glad at heart attempt to create devotion (8)
I believe the answer is:
idolatry
'devotion' is the definition.
('idolatry' can be a synonym of 'devotion')
'wedding vow glad at heart attempt' is the wordplay.
'wedding vow' becomes 'ido' (I've seen this before).
'at heart' says to take the centre.
'attempt' becomes 'try' (I've seen this before).
The central letters of 'glad' are 'la'.
'ido'+'la'+'try'='IDOLATRY'
'to create' is the link.
(Other definitions for idolatry that I've seen before include "Excessive adoration" , "Hero worship" , "Extreme admiration" , "Worship of carved images" , "Worship of godly images" .)