Cryptic crossword clue
Gain an advantage over rotten marshal at once 5,1,5,2

The most common and most likely answer to this crossword clue is the 5 plus 1 plus 5 plus 2 letter word STEAL A MARCH ON

I've seen this crossword clue in The Daily Telegraph
S
T
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A
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M
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N

Buddy explains!

The straight part
We know that "Gain an advantage over" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gain an advantage over = steal a march on.
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.

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