Jointly endorse function for broadcasters (6)

I believe the answer is:

cosign

'jointly endorse' is the definition.
(I know that to cosign is to endorse jointly)

'function for broadcasters' is the wordplay.
'function' becomes 'cosine' (mathematical function).
'for broadcasters' shows a homophone (sound like) (how the word might sound over a broadcast).
'cosine' is a homophone of 'COSIGN'.

I've seen this clue in The Telegraph.

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