The most common and most likely answer to this crossword clue is the 6 letter word SPAYED
I've seen this crossword clue in The Guardian
S
P
A
Y
E
D
Buddy explains!
The straight part
We know that "to be made sexually unproductive" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case to be made sexually unproductive = spayed.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 SPAYED mean?
SPAYED verb- To divine; foretell
- To remove or destroy the ovaries (of an animal) so that it cannot become pregnant.