The most common and most likely answer to this crossword clue is the 8 letter word STROMATA
I've seen this crossword clue in The Guardian
S
T
R
O
M
A
T
A
More clues leading to the result STROMATA
Buddy explains!

The straight part
We know that "Fibrous connective tissues" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fibrous connective tissues = stromata.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 STROMATA mean?
STROMATA noun- The tissue structure of an organ, etc., that serves to support it.