The most common and most likely answer to this crossword clue is the 3 letter word DOS
I've seen this crossword clue in The Times
D
O
S
Buddy explains!

The straight part
We know that "Here sit" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case here sit = dos.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 DOS mean?
DOS noun- A party, celebration, social function.
- A hairdo.
- Something that can or should be done (usually in the phrase dos and don'ts).
- A deed; an act.
- Ado; bustle; stir; to-do; A period of confusion or argument.
- A cheat; a swindler.
- An act of swindling; a fraud or deception.
- A syllable used in solfège to represent the first and eighth tonic of a major scale.
- A party, celebration, social function.
- A hairdo.
- Something that can or should be done (usually in the phrase dos and don'ts).
- A deed; an act.
- Ado; bustle; stir; to-do; A period of confusion or argument.
- A cheat; a swindler.
- An act of swindling; a fraud or deception.
- A syllable used in solfège to represent the first and eighth tonic of a major scale.