Declaration of marriage following admirer (8)
I believe the answer is:
idolater
'admirer' is the definition.
(I've seen this before)
'declaration of marriage following' is the wordplay.
'declaration of marriage' becomes 'ido' (I am not sure about this - if you are sure you should believe this answer much more).
'following' becomes 'later' (later can mean following or subsequent).
'ido'+'later'='IDOLATER'
Can you help me to learn more?
(Other definitions for idolater that I've seen before include "One who worships a false god or graven image" , "Huge admirer" , "Tailored like pagan worshipper" , "great admirer" , "Tailored to worshipper of graven images" .)