Join in mission to follow TikTok or Instagram? (6)

I believe the answer is:

append

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'join' is the definition.
(thesaurus)

I don't understand the rest of the clue.

(Other definitions for append that I've seen before include "provided as add-on" , "Hang on" , "Napped (anag)" , "Add, annex" , "Add (to the bottom of a document)" .)

I've seen this clue in The Times.

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