The most common and most likely answer to this crossword clue is the 3 letter word RAM
R
A
M
More clues leading to the result RAM
Buddy explains!

The straight part
We know that "Drive/farm animal" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case drive/farm animal = ram.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 RAM mean?
RAM noun- A male sheep, typically uncastrated
- A battering ram; a heavy object used for breaking through doors.
- A warship intended to sink other ships by ramming them.
- A piston powered by hydraulic pressure.
- A weight which strikes a blow, in a ramming device such as a pile driver, a steam hammer, a stamp mill.
- To collide with (an object), usually with the intention of damaging it or disabling its function.
- To strike (something) hard, especially with an implement.
- To fill or compact by pounding or driving.
- To thrust during sexual intercourse.
- Rancid, offensive in smell or taste.