Crossword Genius

I'll leave bar, being put on 'nil by mouth' (4)

Ross

I believe the answer is:

oral

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

I can't explain the rest of the clue.

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(Other definitions for oral that I've seen before include "Exam using speech" , "Relating to speech" , "language exam" , "Ant; tourist (dial.)" , "Kind of examination" .)

I've seen this clue in The Telegraph.
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