The most common and most likely answer to this crossword clue is the 5 letter word APTLY
I've seen this crossword clue in The Sunday Telegraph
A
P
T
L
Y
Buddy explains!
The straight part
We know that "that’s how it should be" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case that’s how it should be = aptly.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 APTLY mean?
APTLY adverb- In an apt or suitable manner; fitly; appropriately