Hard to understand a clergyman's retiring in it (6)
I believe the answer is:
severe
'hard' is the definition.
(I know that hard can be written as severe)
'understand a clergyman's retiring in it' is the wordplay.
'understand' becomes 'see' (synonyms).
'a clergyman' becomes 'rev' (short for reverend).
'retiring' shows that the letters should be reversed in order.
'in it' means one lot of letters goes inside another.
'rev' written backwards gives 'ver'.
'see' placed around 'ver' is 'SEVERE'.
'to' is the link.
(Other definitions for severe that I've seen before include "Harsh and stringent" , "Hard to endure" , "Plain, hard" , "Arduous - dangerous" , "Strict, hard" .)