The most common and most likely answer to this crossword clue is the 9 letter word HERMITAGE
I've seen this crossword clue in The Guardian
H
E
R
M
I
T
A
G
E
More clues leading to the result HERMITAGE
- Museum investing millions in cultural inheritance
- What’s left outside front of mean retreat
- What woman's not keen to admit, maybe, crossing front of medieval Italian museum
- Secluded dwelling Mike included in legacy
- Beginning to regret entering the game, I pathetically retreat
- A woman’s journal exhibited at first outside Italian museum?
- Where recluse lives — male in tradition passed down
- Retreat that is concealing means of identification to the east of small island
- Retreat from Moscow's beginning with what's left outside
- Tradition to include prom finale in retreat
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 = hermitage.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 HERMITAGE mean?
HERMITAGE noun- A house or dwelling where a hermit lives.
- A place of seclusion.
- A period of seclusion.