Marred arrangement with one enthusiast (7)
I believe the answer is:
admirer
'enthusiast' is the definition.
(thesaurus)
'marred arrangement with one' is the wordplay.
'arrangement with' indicates anagramming the letters.
'one' becomes 'i' (Roman numeral).
'marred' anagrammed with 'i' gives 'ADMIRER'.
(Other definitions for admirer that I've seen before include "Married to one who thinks highly of" , "Supporter or fan" , "One who thinks highly or romantically of" , "potential suitor" , "Fan or devotee" .)