I will notice them returning about decoration (9)
I believe the answer is:
medallion
'decoration' is the definition.
(synonyms)
'i will notice them returning about' is the wordplay.
'i will' becomes 'I'll' (abbreviation).
'notice' becomes 'ad' (both can mean an advertisement).
'them' becomes 'em' ('em is an informal way of saying 'them').
'returning' says the letters should be written backwards.
'about' becomes 'on' (as in a book 'on' a certain topic).
'ill'+'ad'+'em'='illadem'
'illadem' back-to-front is 'medalli'.
'medalli'+'on'='MEDALLION'
(Other definitions for medallion that I've seen before include "Mad O'Neill wins the award" , "Cut" , "(Commemorative) disc" , "Round pendant" , "Large coin-shaped metal ornament" .)