Cryptic crossword clue
You need 12 to form such a system 10

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

I've seen this crossword clue in Financial Times
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The straight part
We know that "You need 12 to form such a system" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case you need 12 to form such a system = duodecimal.
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

DUODECIMAL noun
  1. A number system that uses twelve as its base.
DUODECIMAL adjective
  1. Of a number, expressed in base twelve.

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