Image of Billy, his third after wedding vow (4)
I believe the answer is:
idol
'image of billy' is the definition.
'idol' can be an answer for 'billy' (I've seen this before). I'm not certain of the 'image of' bit.
'his third after wedding vow' is the wordplay.
'his third' becomes 'l' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'after' is a charade indicator (letters next to each other).
'wedding vow' becomes 'I do' ('I do' is part of the traditional wedding vows).
'l' after 'ido' is 'IDOL'.
Can you help me to learn more?
(Other definitions for idol that I've seen before include "Heathen god" , "Object of worship (or love?)" , "Film star" , "Something worshipped" , "Someone much loved" .)