Crossword Genius

The third to leave party? That's annoying! (3)

Ross

I believe the answer is:

bah

'that's annoying' is the definition.
(expression of annoyance)

I cannot understand how the remainder of the clue works.

(Other definitions for bah that I've seen before include "Expression of disgust or contempt" , "Exclamation of disgust or annoyance" , "''... Humbug'' Scrooge's cry" , "Exclamation of contempt or disgust - Scrooge used to say it" , "Appalling!" .)

I've seen this clue in The Times.
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