Reformist Ann could turn out to be a pioneer (12)
I believe the answer is:
frontiersman
'a pioneer' is the definition.
(I know that frontiersman is a type of pioneer)
'reformist ann could turn out to be' is the wordplay.
'could turn out to be' indicates an anagram.
'reformist'+'ann'='reformistann'
'reformistann' is an anagram of 'FRONTIERSMAN'.
(Other definitions for frontiersman that I've seen before include "pioneer" , "Border dweller" , "his position is borderline" .)