The most common and most likely answer to this crossword clue is the 7 plus 1 letter word PLASMA –
I've seen this crossword clue in The Daily Telegraph
P
L
A
S
M
A
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“
Buddy explains!
The straight part
We know that "Fluid" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fluid = plasma –.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 PLASMA – mean?
PLASMA – noun- A state of matter consisting of partially ionized gas and electrons
- A clear component of blood or lymph containing fibrin
- Blood plasma, free of suspended cells, used in transfusions
- A variety of green quartz, used in ancient times for making engraved ornaments.
- A mixture of starch and glycerin, used as a substitute for ointments.
- A visual effect in which cycles of changing colours are warped in various ways to give the illusion of liquid organic movement.