The most common and most likely answer to this crossword clue is the 12 letter word REQUIREMENTS
I've seen this crossword clue in The Daily Telegraph
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More clues leading to the result REQUIREMENTS
Buddy explains!
The straight part
We know that "demands" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case demands = requirements.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 REQUIREMENTS mean?
REQUIREMENTS noun- A necessity or prerequisite; something required or obligatory. Its adpositions are generally of in relation to who or what has given it, on in relation to whom or what it is given to, and for in relation to what is required.
- Something asked.
- A statement (in domain specific terms) which specifies a verifiable constraint on an implementation that it shall undeniably meet or (a) be deemed unacceptable, or (b) result in implementation failure, or (c) result in system failure.