Great unorthodox priest's last sermon due (10)
I believe the answer is:
tremendous
'great' is the definition.
(thesaurus)
'unorthodox priest's last sermon due' is the wordplay.
'unorthodox' is an anagram indicator.
'last' says to take the final letters.
The last letter of 'priest' is 't'.
't'+'sermon'+'due'='tsermondue'
'tsermondue' is an anagram of 'TREMENDOUS'.
(Other definitions for tremendous that I've seen before include "Wonderful" , "Splendid!" , "Terrific" , "Excellent; very great" , "large" .)