The most common and most likely answer to this crossword clue is the 4 plus 5 plus 2 plus 3 plus 4 letter word FAIR CRACK OF THE WHIP
I've seen this crossword clue in The Guardian
F
A
I
R
C
R
A
C
K
O
F
T
H
E
W
H
I
P
Buddy explains!
The straight part
We know that "a reasonable chance" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a reasonable chance = fair crack of the whip.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.