The most common and most likely answer to this crossword clue is the 7 letter word AIRLOCK
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Buddy explains!
The straight part
We know that "what’s blocking pipe" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what’s blocking pipe = airlock.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 AIRLOCK mean?
AIRLOCK noun- A small amount of air in a pipe that stops liquid flowing through it
- A sealed, airtight chamber, such as in a manned spacecraft or submarine, used to provide access to and from the sealed area without allowing air out or water in.