The most common and most likely answer to this crossword clue is the 3 letter word HIS
I've seen this crossword clue in The Guardian
H
I
S
More clues leading to the result HIS
Buddy explains!
The straight part
We know that "not hers" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not hers = his.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 HIS mean?
HIS pronoun- Honorific alternative letter-case form of his, sometimes used when referring to God or another important figure who is understood from context.
- That which belongs to him; the possessive case of he, used without a following noun.
- The word "hi" used as a greeting.