Supporter lost ground in a sport (7)
I believe the answer is:
apostle
'supporter' is the definition.
(apostle is a kind of supporter)
'lost ground in a sport' is the wordplay.
'ground' indicates an anagram ('grind' the letters into a new form).
'in' is an insertion indicator.
'a sport' becomes 'ape' (I am not sure about this - if you are sure you should believe this answer much more).
'lost' with letters rearranged gives 'ostl'.
'ostl' put within 'ape' is 'APOSTLE'.
(Other definitions for apostle that I've seen before include "Divine messenger" , "Early Christian missionary" , "To lapse (anag) - ardent supporter of a cause" , "One of the biblical twelve (7)well don" , "Missionary or evangelist" .)