Priest forces son to reform (9)
I believe the answer is:
confessor
'priest' is the definition.
(confessor is a kind of priest)
'forces son to reform' is the wordplay.
'to reform' indicates an anagram (letters in a new form).
'forces'+'son'='forcesson'
'forcesson' with letters rearranged gives 'CONFESSOR'.
(Other definitions for confessor that I've seen before include "Priest who hears sins" , "One listens to the sins of others" , "Priest who hears admission of faults" , "One who makes a declaration of faith" , "man who tells all?" .)