The most common and most likely answer to this crossword clue is the 6 plus 4 letter word DOUBLE TIME
I've seen this crossword clue in Financial Times
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More clues leading to the result DOUBLE TIME
Buddy explains!
The straight part
We know that "What fast-moving troops do" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what fast-moving troops do = double time.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 DOUBLE TIME mean?
DOUBLE TIME verb- To march at double time.
- To order troops to march at double time.