The most common and most likely answer to this crossword clue is the 7 letter word REQUIRE
I've seen this crossword clue in Independent
R
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Buddy explains!

The straight part
We know that "Demand" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case demand = require.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 REQUIRE mean?
REQUIRE verb- To ask (someone) for something; to request.
- To demand, to insist upon (having); to call for authoritatively.
- Naturally to demand (something) as indispensable; to need, to call for as necessary.
- To demand of (someone) to do something.