Powerful preacher is on exam schedule (10)
I believe the answer is:
revivalist
'powerful preacher' is the definition.
'revivalist' can be an answer for 'preacher' (revivalist is a kind of preacher). I'm not sure about the 'powerful' bit.
'on exam schedule' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'exam' becomes 'viva' (short for 'viva voce', an oral examination).
'schedule' becomes 'list' (I've seen this in other clues).
're'+'viva'+'list'='REVIVALIST'
'is' acts as a link.
Can you help me to learn more?
(Another definition for revivalist that I've seen is " Preacher".)