As a concert piece, it can't fail (4)

I believe the answer is:

cert

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

'it can't fail' is the definition.
The definition suggests a singular noun which matches the answer.

'as a concert piece' is the wordplay.
'CERT' is hidden in amongst the letters.
But, I am uncertain how the hidden word is indicated.

Can you help me to learn more?

(Other definitions for cert that I've seen before include "Classification" , "Inevitable event" , "One sure to win (slang)" , "Sure thing (that could be dead!)" , "Sure thing, in short" .)

I've seen this clue in the Evening Standard.

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