The most common and most likely answer to this crossword clue is the 8 letter word MEGADOSE
I've seen this crossword clue in The Times
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Buddy explains!
The straight part
We know that "Amount of medicine" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case amount of medicine = megadose.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 MEGADOSE mean?
MEGADOSE noun- A dose of drug or vitamin far exceeding the normal or recommended amount, and usually given intentionally. Compare overdose.
- To dose (a patient) with a very large amount of a drug.
- To dose with a very large amount of (a drug).