The most common and most likely answer to this crossword clue is the 10 letter word CORNCOCKLE
C
O
R
N
C
O
C
K
L
E
Buddy explains!
The straight part
We know that "a threat to the harvest" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a threat to the harvest = corncockle.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 CORNCOCKLE mean?
CORNCOCKLE noun- Any of several species of flowering plants, of the genus Agrostemma, in the Caryophyllaceae family.