Cryptic crossword clue
Promises phosphorus-free new chemical compounds 7

The most common and most likely answer to this crossword clue is the 7 letter word ISOMERS

I've seen this crossword clue in The Daily Telegraph
I
S
O
M
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S

Buddy explains!

The straight part
We know that "chemical compounds" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case chemical compounds = isomers.
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 ISOMERS mean?

ISOMERS noun
  1. Any of two or more compounds with the same molecular formula but with different structure.
  2. Any of two or more atomic nuclei with the same mass number and atomic number but with different radioactive properties.

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