The most common and most likely answer to this crossword clue is the 5 letter word PILED
I've seen this crossword clue in The Guardian
P
I
L
E
D
More clues leading to the result PILED
Buddy explains!

The straight part
We know that "in a heap" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in a heap = piled.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 PILED mean?
PILED verb- (often used with the preposition "up") To lay or throw into a pile or heap; to heap up; to collect into a mass; to accumulate
- To cover with heaps; or in great abundance; to fill or overfill; to load.
- To add something to a great number.
- (of vehicles) To create a hold-up.
- To place (guns, muskets, etc.) together in threes so that they can stand upright, supporting each other.
- To drive piles into; to fill with piles; to strengthen with piles.
- To give a pile to; to make shaggy.
- (iron manufacturing) Formed from a pile or fagot.
- Having a pile or point; pointed.
- Having a pile or nap.