The most common and most likely answer to this crossword clue is the 6 letter word COTTER
I've seen this crossword clue in Financial Times
C
O
T
T
E
R
Buddy explains!

The straight part
We know that "old farm worker" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old farm worker = cotter.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 COTTER mean?
COTTER noun- A pin or wedge inserted through a slot to hold machine parts together.
- A cotter pin.
- To fasten with a cotter.
- A peasant who performed labour in exchange for the right to live in a cottage.