Cavities in human trachea (5)
I believe the answer is:
antra
'cavities' is the definition.
(I've seen this before)
'in human trachea' is the wordplay.
'in' indicates a hidden word.
'ANTRA' can be found hidden inside 'human trachea'.
(Other definitions for antra that I've seen before include "Chambers" , "Hollow spaces" , "Cavities - a rant (anag)" , "Natural cavities in bone; ratan (anag.)" , "Cave!" .)