The most common and most likely answer to this crossword clue is the 5 plus 7 letter word RURAL DEANERY
I've seen this crossword clue in The Guardian
R
U
R
A
L
D
E
A
N
E
R
Y
Buddy explains!
The straight part
We know that "Clerical office" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case clerical office = rural deanery.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.