Infilling suspect cavity (5)

I believe the answer is:

sinus

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

I cannot understand how the rest of the clue works.

(Other definitions for sinus that I've seen before include "Hollow space" , "Cavity linked to nose" , "snotty enclosure" , "Head space" , "Cavity in skull connecting with nostrils" .)

I've seen this clue in The Guardian.

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