The most common and most likely answer to this crossword clue is the 5 plus 3 letter word DON’T ASK
I've seen this crossword clue in The Guardian
D
O
N
â
€
™
T
A
S
K
Buddy explains!
The straight part
We know that "that is not worth explaining" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case that is not worth explaining = don’t ask.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.