The most common and most likely answer to this crossword clue is the 3 plus 6 plus 2 plus 1 plus 5 letter word THE ANSWER IS A LEMON
I've seen this crossword clue in The Times
T
H
E
A
N
S
W
E
R
I
S
A
L
E
M
O
N
Buddy explains!
The straight part
We know that "Don't be unreasonable!" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case don't be unreasonable! = the answer is a lemon.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.