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'.
However, I am not sure how this 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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