They may conceal terrible evils (5)
I believe the answer is:
veils
'they may conceal' is the definition.
The definition suggests an adverb but the answer is not.
'terrible evils' is the wordplay.
'terrible' is an anagram indicator.
'evils' anagrammed gives 'VEILS'.
Can you help me to learn more?
(Other definitions for veils that I've seen before include "Masks" , "Garments to cover head and face" , "Covers or partly conceals - bride's face?" , "See 20" , "coverage" .)