The most common and most likely answer to this crossword clue is the 6 letter word WETHER
I've seen this crossword clue in The Guardian
W
E
T
H
E
R
Buddy explains!

The straight part
We know that "sheep" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sheep = wether.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 WETHER mean?
WETHER noun- A castrated buck goat.
- A castrated ram.
- To castrate a male sheep or goat.
- The short term state of the atmosphere at a specific time and place, including the temperature, relative humidity, cloud cover, precipitation, wind, etc.
- Unpleasant or destructive atmospheric conditions, and their effects.
- The direction from which the wind is blowing; used attributively to indicate the windward side.
- A situation.
- A storm; a tempest.
- A light shower of rain.