One following three, and in order (8)
I believe the answer is:
adherent
'one following' is the definition.
'three and in order' is the wordplay.
'in order' is an anagram indicator (the letters in a new order).
'three'+'and'='threeand'
'threeand' anagrammed gives 'ADHERENT'.
(Other definitions for adherent that I've seen before include "Follower, partisan" , "Supporter, fan" , "Freak" , "Dedicated supporter" , "Loyal follower, one who sticks" .)