Supporter's three and out (8)
I believe the answer is:
adherent
'supporter's' is the definition.
(I've seen this before)
'three and out' is the wordplay.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'three'+'and'='threeand'
'threeand' is an anagram of 'ADHERENT'.
(Other definitions for adherent that I've seen before include "Sticking; partisan" , "supporter of party" , "Loyal follower, one who sticks" , "One supporting" , "Supporter; sticking" .)