The most common and most likely answer to this crossword clue is the 12 letter word HAEMORRHOIDS
I've seen this crossword clue in The Times
H
A
E
M
O
R
R
H
O
I
D
S
Buddy explains!

The straight part
We know that "piles" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case piles = haemorrhoids.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 HAEMORRHOIDS mean?
HAEMORRHOIDS noun- A pathological condition caused by painful masses of dilated veins in swollen anal tissue.
- (often in the plural) An engorged, dilated and easily broken varicosity in the perianal area, often accompanied by intense itching and throbbing pain: piles.