Where an oral surgeon might be looking unhappy (4,2,3,5)

I believe the answer is:

down in the mouth

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'unhappy' is the definition.
(I've seen this before)

I cannot understand how the rest of the clue works.

(Other definitions for down in the mouth that I've seen before include "Not happy" , "consequence of eating unprepared duck?" , "feathers, orally" , "then spitting feathers?" , "Hangdog" .)

I've seen this clue in The Hindu.

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