The most common and most likely answer to this crossword clue is the 4 letter word SERA
S
E
R
A
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The straight part
We know that "body fluids" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case body fluids = sera.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 SERA mean?
SERA noun- The clear yellowish liquid obtained upon separating whole blood into its solid and liquid components after it has been allowed to clot.
- Blood serum from the tissues of immunized animals, containing antibodies and used to transfer immunity to another individual, called antiserum.
- A watery liquid from animal tissue, especially one that moistens the surface of serous membranes or that is exuded by such membranes when they become inflamed, such as in edema or a blister.
- The watery portion of certain animal fluids like blood, milk, etc; whey.
- (skincare) An intensive moisturising product to be applied after cleansing but before a general moisturiser.