The most common and most likely answer to this crossword clue is the 13 letter word DISENGAGEMENT
I've seen this crossword clue in The Daily Telegraph
D
I
S
E
N
G
A
G
E
M
E
N
T
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 = disengagement.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 DISENGAGEMENT mean?
DISENGAGEMENT noun- Release or detachment from a physical situation or other involvement.
- The separation or release of a chemical.
- Leisure; relief from responsibilities or onerous activities.
- Withdrawal from combat, confrontation, or the assertion of influence.
- Termination of an agreement to be married.
- A circular movement of the blade that blocks an opponent's parry.
- The emergence of the fetus from the birth canal.