It’s hard to understand a clergyman’s retiring in it (6)
I believe the answer is:
severe
'it's hard to' is the definition.
(I've seen this before)
'understand a clergyman's retiring in it' is the wordplay.
'understand' becomes 'see' (synonyms).
'a clergyman' becomes 'rev' (I've seen this before).
'retiring' says the letters should be written in reverse.
'in it' is an insertion indicator.
'rev' backwards is 'ver'.
'see' placed around 'ver' is 'SEVERE'.
(Other definitions for severe that I've seen before include "Unsparing and uncompromising" , "Drastic" , "Grave" , "Draconian" , "Harsh and stringent" .)