The most common and most likely answer to this crossword clue is the 9 letter word HYPOTONIC
I've seen this crossword clue in Independent
H
Y
P
O
T
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C
More clues leading to the result HYPOTONIC
Buddy explains!
The straight part
We know that "lacking muscle tone" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lacking muscle tone = hypotonic.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 HYPOTONIC mean?
HYPOTONIC adjective- (of a solution) Having a lower osmotic pressure than another.
- Showing less than normal tension in the muscles or muscle tissue.