Cryptic crossword clue
Blue substance ?- is this what officers get paid after dark, did you say? 6,7

The most common and most likely answer to this crossword clue is the 6 plus 7 letter word COPPER NITRATE

I've seen this crossword clue in The Guardian
C
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Buddy explains!

The straight part
We know that "Blue substance" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case blue substance = copper nitrate.
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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