The most common and most likely answer to this crossword clue is the 5 letter word ADAWS
A
D
A
W
S
Buddy explains!
The straight part
We know that "cows out of service" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case cows out of service = adaws.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.