The most common and most likely answer to this crossword clue is the 7 letter word CONVENT
C
O
N
V
E
N
T
More clues leading to the result CONVENT
- Study archdeacon starting tombola in religious quarters
- Group of nuns furnishing prisoner with means of escape
- Fraud opening religious community
- Criminal let out from closed institution
- Meeting barring one — on at sister's place
- Dupe, escape and retreat
- Study opening in religious establishment
- Where order is kept to release criminal at first
- Such a school may be happy to have a change of heart
Buddy explains!

The straight part
We know that "retreat" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case retreat = convent.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.
What does CONVENT mean?
CONVENT noun- A religious community whose members (especially nuns) live under strict observation of religious rules and self-imposed vows.
- The buildings and pertaining surroundings in which such a community lives.
- A Christian school.
- A gathering of people lasting several days for the purpose of discussing or working on topics previously selected.
- A coming together; a meeting.
- To call before a judge or judicature; to summon; to convene.
- To meet together; to concur.
- To be convenient; to serve.