Marriage promise in due course for admirer (8)

I believe the answer is:

idolater

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'for admirer' is the definition.
(I've seen this before)

'marriage promise in due course' is the wordplay.
'marriage promise' becomes 'ido' (I can't explain this - if you can you should give a lot more credence to this answer).
'in due course' becomes 'later' (I've seen this before).
'ido'+'later'='IDOLATER'

Can you help me to learn more?

(Other definitions for idolater that I've seen before include "Besotted admirer" , "No orthodox believer" , "Tailored for image worshipper" , "One admires" , "A person who worships idols" .)

I've seen this clue in The Sun.

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