The most common and most likely answer to this crossword clue is the 8 letter word MARSUPIA
I've seen this crossword clue in The Times
M
A
R
S
U
P
I
A
More clues leading to the result MARSUPIA
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The straight part
We know that "Natural pockets" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case natural pockets = marsupia.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 MARSUPIA mean?
MARSUPIA noun- The external pouch in which female marsupials rear and feed the young.
- A brood pouch in some fishes, crustaceans and insects in the family Monophlebidae.