A pioneer of a reform movement:

Solution (By Examveda Team)

The one word substitution is Apostle.

Apothecary : a person who prepared and sold medicines and drugs.
Apotheosis : the highest point in the development of something; a culmination or climax.
Renegade : a person who deserts and betrays an organization, country, or set of principles.
Apostle : a vigorous and pioneering advocate or supporter of a particular policy, idea, or cause.