The most common and most likely answer to this crossword clue is the 9 letter word BACKWARDS
I've seen this crossword clue in The Times
B
A
C
K
W
A
R
D
S
Buddy explains!

The straight part
We know that "in retreat?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in retreat? = backwards.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 BACKWARDS mean?
BACKWARDS adjective- Oriented toward the back.
- Reversed.
- Behind current trends or technology.
- Clumsy, inept, or inefficient, especially in learning.
- Toward the back.
- In the opposite direction to usual.
- In a manner such that the back precedes the front.
- Towards the past; ago