Cavity appears in useless bottles (5)
I believe the answer is:
sinus
'cavity' is the definition.
(sinus is a kind of cavity)
'appears in useless bottles' is the wordplay.
'bottles' says the answer is hidden in the clue (bottling is a kind of containing).
'SINUS' is hidden within 'appears in useless'.
(Other definitions for sinus that I've seen before include "Passage in the head" , "Hollow space" , "Cavity in skull connecting with nostrils" , "Space in temple" , "Hole in the head" .)