The most common and most likely answer to this crossword clue is the 7 letter word WINDROW
I've seen this crossword clue in The Guardian
W
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N
D
R
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W
More clues leading to the result WINDROW
Buddy explains!

The straight part
We know that "stacks of hay" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case stacks of hay = windrow.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 WINDROW mean?
WINDROW noun- A row of cut grain or hay allowed to dry in a field.
- A line of leaves etc heaped up by the wind.
- A similar streak of seaweed etc on the surface of the sea formed by Langmuir circulation.
- A line of snow or gravel left behind by the edge of a snowplow’s or grader’s blade.
- The green border of a field, dug up in order to carry the earth on other land to mend it.
- To arrange (e.g. new-made hay) in lines or windrows.