Cryptic crossword clue
Failing old firm in security almost unknown 10

The most common and most likely answer to this crossword clue is the 10 letter word DEFICIENCY

I've seen this crossword clue in The Times
D
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The straight part
We know that "Failing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case failing = deficiency.
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.

Crossword Answer definitions

DEFICIENCY noun
  1. Inadequacy or incompleteness.
  2. An insufficiency, especially of something essential to health.
  3. The amount by which the number of double points on a curve is short of the maximum for curves of the same degree.
  4. The codimension of a linear system in the corresponding complete linear system.

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