The most common and most likely answer to this crossword clue is the 9 letter word DIGITALIN
I've seen this crossword clue in The Times
D
I
G
I
T
A
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N
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The straight part
We know that "heart drug" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case heart drug = digitalin.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 DIGITALIN mean?
DIGITALIN noun- Any of a mixture of glycosides, extracted from the foxglove plant, that are used as cardiotonics.