Covering evil operation (4)

I believe the answer is:

veil

I'm a little stuck... Click here to teach me more about this clue!

'covering' is the definition.
(veil is a kind of covering)

'evil operation' is the wordplay.
'evil' anagrams to 'VEIL'.
Though, I'm not sure how the anagram is indicated.

Can you help me to learn more?

(Other definitions for veil that I've seen before include "Transparent face cover" , "Nun's headdress" , "Fabric face covering" , "Nun's head-covering" , "Yashmak, e.g." .)

I've seen this clue in The Mirror.

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