Slogans sides put out? They are hollow (5)

I believe the answer is:

antra

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'they are hollow' is the definition.
(plural of antrum, a cavity in the body)

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for antra that I've seen before include "Natural hollows" , "Chambers" , "Cave!" , "Cavities - a rant (anag)" , "Natural cavities in bone; ratan (anag.)" .)

I've seen this clue in The Times.

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