Some fanatical vindictive churchman of yesteryear (6)
I believe the answer is:
calvin
'churchman of yesteryear' is the definition.
(16th-century theologian John Calvin)
'some fanatical vindictive' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'CALVIN' is hidden in the letters of 'fanatical vindictive'.
(Other definitions for calvin that I've seen before include "Reformation theologian" , "John -, sixteenth century French-born theologian" , "John . . . . . ., French Protestant religious reformer of 16th century (6)" , "Christian theologian" , "Chief expositor of Presbyterian beliefs" .)