At home, a Right Reverend mixed up 'arse' and 'behind' (2,7)
I believe the answer is:
in arrears
'behind' is the definition.
(thesaurus)
'at home a right reverend mixed up arse' is the wordplay.
'at home' becomes 'in' ('in' can be similar in meaning to 'at home').
'right reverend' becomes 'rr'.
'mixed up' indicates anagramming the letters.
'arse' is an anagram of 'ears'.
'in'+'a'+'rr'+'ears'='IN ARREARS'
'and' acts as a link.
(Other definitions for in arrears that I've seen before include "Behindhand (with rent)" , "Owing" , "Behind with payments" , "Late with payments" , "Rare rains beyond the due time" .)