The most common and most likely answer to this crossword clue is the 5 letter word POUCH
I've seen this crossword clue in The Times
P
O
U
C
H
More clues leading to the result POUCH
Buddy explains!

The straight part
We know that "body cavity" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case body cavity = pouch.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 POUCH mean?
POUCH noun- A small bag usually closed with a drawstring.
- A pocket in which a marsupial carries its young.
- Any pocket or bag-shaped object, such as a cheek pouch.
- A protuberant belly; a paunch.
- A cyst or sac containing fluid.
- A silicle, or short pod, as of the shepherd's purse.
- A bulkhead in the hold of a vessel, to prevent grain etc. from shifting.
- To enclose within a pouch.
- To transport within a pouch, especially a diplomatic pouch.
- (of fowls and fish) To swallow.
- To pout.
- To pocket; to put up with.