Admirer's one-party following (8)
I believe the answer is:
idolater
'admirer's' is the definition.
(I've seen this before)
'one-party following' is the wordplay.
'one' becomes 'i' (Roman numeral).
'party' becomes 'do' (do is a kind of party).
'following' becomes 'later' (I've seen this before).
'i'+'do'+'later'='IDOLATER'
(Other definitions for idolater that I've seen before include "Tailored to worshipper of graven images" , "devotee" , "One worshipping graven image" , "A person who worships idols" , "Tailored for image worshipper" .)