Cavity, small, not fully active (5)

I believe the answer is:

sinus

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'cavity' is the definition.
(I know that sinus is a type of cavity)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for sinus that I've seen before include "Hole in the head" , "Airspace" , "Air-filled cavity of the skull" , "Cavity in the facial bones" , "Nasal cavity" .)

I've seen this clue in The Independent.

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