Mad when last of guests leaves? That’s ridiculous (5)

I believe the answer is:

inane

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'ridiculous' is the definition.
(thesaurus)

I don't understand the rest of the clue.

(Other definitions for inane that I've seen before include "unimaginative" , "Daft" , "Silly, witless" , "Absurd" , "Silly or senseless" .)

I've seen this clue in The Times.

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