It's the one fault that spoils Tim Reed (7)
I believe the answer is:
demerit
'it's the one fault' is the definition.
(synonyms)
'spoils tim reed' is the wordplay.
'spoils' indicates an anagram.
'tim'+'reed'='timreed'
'timreed' with letters rearranged gives 'DEMERIT'.
'that' is the link.
(Other definitions for demerit that I've seen before include "Fault, defect" , "Something deserving censure" , "Black mark" , "Feature deserving censure" , "Mark given for a fault" .)