The most common and most likely answer to this crossword clue is the 6 plus 3 letter word HIDDEN GEM
I've seen this crossword clue in Independent
H
I
D
D
E
N
G
E
M
Buddy explains!
The straight part
We know that "unexpected treat" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unexpected treat = hidden gem.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.