The most common and most likely answer to this crossword clue is the 8 letter word SCYTHING
I've seen this crossword clue in The Times
S
C
Y
T
H
I
N
G
More clues leading to the result SCYTHING
Buddy explains!

The straight part
We know that "old harvester’s job?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old harvester’s job? = scything.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 SCYTHING mean?
SCYTHING verb- To use a scythe.
- To cut with a scythe.
- To cut off as with a scythe; to mow.
- To attack or injure as if cutting.
- The act of using a scythe.
- The mown material left after using a scythe.