The most common and most likely answer to this crossword clue is the 12 letter word OVERCHARGING
I've seen this crossword clue in The Times
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Buddy explains!

The straight part
We know that "Demanding too much" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case demanding too much = overcharging.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 OVERCHARGING mean?
OVERCHARGING noun- The act or process of charging excessively
- To charge (somebody) more money than the correct amount or to surpass a certain limit while charging a bill.
- To continue to charge (an electrical device) beyond its capacity.
- To charge (someone) with an inflated number or degree of legal charges (for example, charging them with a more serious crime than was committed); to upcharge.
- To charge or load too heavily; to burden; to oppress.
- To fill too full; to crowd.
- To exaggerate.