The most common and most likely answer to this crossword clue is the 3 letter word HON
I've seen this crossword clue in The Daily Telegraph
H
O
N
Buddy explains!

The straight part
We know that "sweetie" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sweetie = hon.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 HON mean?
HON noun- (used only to address someone) Honey, sweetheart, a term of endearment; a friendly term of address.
- ; cheering a sports team, especially a GAA team; exhortation or encouragement come on; congratulations well done, bravo