Cryptic crossword clue
Dirty ancient moat in ruins 11

The most common and most likely answer to this crossword clue is the 11 letter word CONTAMINATE

I've seen this crossword clue in The Daily Telegraph
C
O
N
T
A
M
I
N
A
T
E

Buddy explains!

The straight part
We know that "Dirty" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dirty = contaminate.
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 CONTAMINATE mean?

CONTAMINATE verb
  1. To make something dangerous or toxic by introducing impurities or foreign matter.
  2. To soil, stain, corrupt, or infect by contact or association.
  3. To make unfit for use by the introduction of unwholesome or undesirable elements.
  4. To infect, often with bad objects

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