The most common and most likely answer to this crossword clue is the 7 letter word EMPYEMA
E
M
P
Y
E
M
A
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The straight part
We know that "pus pocket" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case pus pocket = empyema.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 EMPYEMA mean?
EMPYEMA noun- A collection of pus within a naturally existing anatomical cavity (as opposed to an abscess, which occurs in a newly formed cavity).