The most common and most likely answer to this crossword clue is the 3 plus 4 plus 3 letter word THE EVIL EYE
I've seen this crossword clue in The Guardian
T
H
E
E
V
I
L
E
Y
E
Buddy explains!
The straight part
We know that "look likely to be frightening" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case look likely to be frightening = the evil eye.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.