Cryptic crossword clue
Fixer to note: this is lacking muscle tone 9

The most common and most likely answer to this crossword clue is the 9 letter word HYPOTONIC

I've seen this crossword clue in The Guardian
H
Y
P
O
T
O
N
I
C

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
  1. (of a solution) Having a lower osmotic pressure than another.
  2. Showing less than normal tension in the muscles or muscle tissue.

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