The most common and most likely answer to this crossword clue is the 9 letter word OVERTAXES
I've seen this crossword clue in The Guardian
O
V
E
R
T
A
X
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S
Buddy explains!

The straight part
We know that "Demands too much" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case demands too much = overtaxes.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 OVERTAXES mean?
OVERTAXES verb- To tax to an excessive degree
- To overburden