The most common and most likely answer to this crossword clue is the 2 plus 5 plus 5 letter word DO ONE’S WORST
I've seen this crossword clue in The Guardian
D
O
O
N
E
â
€
™
S
W
O
R
S
T
Buddy explains!
The straight part
We know that "Be t/horoughly malicious" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case be t/horoughly malicious = do one’s worst.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.