The most common and most likely answer to this crossword clue is the 7 letter word ACCRETE
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More clues leading to the result ACCRETE
Buddy explains!

The straight part
We know that "Pile up" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case pile up = accrete.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 ACCRETE mean?
ACCRETE verb- To grow together, combine; to fuse.
- To adhere; to grow or to be added to gradually.
- To make adhere; to add; to make larger or more, as by growing.
- Characterized by accretion; made up
- Grown together