The most common and most likely answer to this crossword clue is the 3 letter word OHO
I've seen this crossword clue in The Times
O
H
O
Buddy explains!
The straight part
We know that "There’s a surprise" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case there’s a surprise = oho.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 OHO mean?
OHO interjection- Expressing surprise or gloating realisation; aha.