Having beef, vanilla slices next (11)

I believe the answer is:

complaining

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'next' is the definition.
Both the definition and answer are adjectives. Maybe there's an association between them I don't understand?

'having beef vanilla slices' is the wordplay.
'having beef' becomes 'coming' (I can't justify this - if you can you should believe this answer much more).
'vanilla' becomes 'plain' (I've seen this in another clue).
'slices' is an insertion indicator (inserted letters slice their way in).
'coming' going around 'plain' is 'COMPLAINING'.

Can you help me to learn more?

(Another definition for complaining that I've seen is " Expressing dissatisfaction".)

I've seen this clue in The Times and The Sunday Times.

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