The most common and most likely answer to this crossword clue is the 12 letter word BACKTRACKING
I've seen this crossword clue in The Times
B
A
C
K
T
R
A
C
K
I
N
G
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 = backtracking.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 BACKTRACKING mean?
BACKTRACKING verb- To retrace one's steps.
- To repeat or review work already done.
- To taxi down an active runway in the opposite direction to that being used for takeoff.
- To exercise a racehorse around the racetrack in the opposite direction to that in which races are run.
- (countable, uncountable) The act of one who, or that which, backtracks; a retracing of one's steps.
- The usage of a runway as a taxiway, especially at private strips and smaller airports.
- The act of building all possible solutions to a problem incrementally, abandoning any candidate solution if it cannot lead to a valid solution.