The most common and most likely answer to this crossword clue is the 5 plus 3 letter word CHEER UP!
I've seen this crossword clue in Financial Times
C
H
E
E
R
U
P
!
Buddy explains!
The straight part
We know that "be less miserable!" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case be less miserable! = cheer up!.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.