The most common and most likely answer to this crossword clue is the 5 letter word EVILS
I've seen this crossword clue in The Guardian
E
V
I
L
S
Buddy explains!
The straight part
We know that "what they are capable of" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what they are capable of = evils.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 EVILS mean?
EVILS noun- Moral badness; wickedness; malevolence; the forces or behaviors that are the opposite or enemy of good.
- Something which impairs the happiness of a being or deprives a being of any good; something which causes suffering of any kind to sentient beings; harm; injury; mischief.
- A malady or disease; especially in the phrase king's evil (scrofula).