The most common and most likely answer to this crossword clue is the 10 letter word EPITHELIUM
I've seen this crossword clue in Financial Times
E
P
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T
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The straight part
We know that "tissue" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case tissue = epithelium.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 EPITHELIUM mean?
EPITHELIUM noun- A membranous tissue composed of one or more layers of cells which forms the covering of most internal and external surfaces of the body and its organs: internally including the lining of vessels and other small cavities, and externally being the skin.