He ranted about disciple (8)
I believe the answer is:
adherent
'disciple' is the definition.
(I know that disciple can be written as adherent)
'he ranted about' is the wordplay.
'about' is an anagram indicator.
'he'+'ranted'='heranted'
'heranted' is an anagram of 'ADHERENT'.
(Other definitions for adherent that I've seen before include "sticker" , "Loyal follower, one who sticks" , "One believes in and helps spread the doctrine of another" , "Supporter, follower" , "Devotee" .)