The most common and most likely answer to this crossword clue is the 7 plus 3 plus 4 letter word AGAINST ALL ODDS
I've seen this crossword clue in The Guardian
A
G
A
I
N
S
T
A
L
L
O
D
D
S
Buddy explains!
The straight part
We know that "Most unexpectedly" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case most unexpectedly = against all odds.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.