The most common and most likely answer to this crossword clue is the 4 plus 2 letter word FAIR DO
I've seen this crossword clue in The Times
F
A
I
R
D
O
Buddy explains!
The straight part
We know that "That's reasonable" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case that's reasonable = fair do.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.