The most common and most likely answer to this crossword clue is the 7 letter word TURN-UP
I've seen this crossword clue in The Times
T
U
R
N
-
U
P
Buddy explains!

The straight part
We know that "a surprise" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a surprise = turn-up.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 TURN-UP mean?
TURN-UP noun- Fabric turned up at the bottom of trousers to make them shorter.
- A fight or disturbance.
- The next card taken from the top of a pack of cards and displayed.
- A stroke of good luck; something that appears unexpectedly; especially in turn up for the book.