Strict clergyman turns up in diocese (6)
I believe the answer is:
severe
'strict' is the definition.
(thesaurus)
'clergyman turns up in diocese' is the wordplay.
'clergyman' becomes 'rev' (I've seen this before).
'turns up' says the letters should be written in reverse.
'in' means one lot of letters goes inside another.
'diocese' becomes 'see' (I've seen this before).
'rev' backwards is 'ver'.
'ver' placed within 'see' is 'SEVERE'.
(Other definitions for severe that I've seen before include "Arduous - dangerous" , "Draconian" , "Harsh, rigorous" , "Hard to endure" , "Austere, serious" .)