The usefulness of a veil? (5)

I believe the answer is:

avail

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'the usefulness' is the definition.
(avail is a kind of usefulness)

'a veil?' is the wordplay.
'veil?' becomes 'vail' (I am not sure about this - if you are sure you should believe this answer much more).
'a'+'vail'='AVAIL'

'of' is the link.

Can you help me to learn more?

(Other definitions for avail that I've seen before include "Medal (informal)" , "Use to one's advantage" , "Be of service to" , "See 3" , "Be of value to" .)

I've seen this clue in the Evening Standard.

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