They're sure to contribute to the concert season (5)

I believe the answer is:

certs

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'they're sure' is the definition.
The definition suggests an adverb but the answer is not.

'contribute to the concert season' is the wordplay.
'contribute to' indicates the answer is hidden within the clue (I've seen 'contributing to' mean this (the answer contributes to part of the clue )).
'CERTS' can be found hidden inside 'concert season'.

'to' is the link.

Can you help me to learn more?

(Another definition for certs that I've seen is " Crest (anag.)".)

I've seen this clue in the Evening Standard.

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