Comeback of star is hollow (5)
I believe the answer is:
sinus
'hollow' is the definition.
(thesaurus)
'comeback of star is' is the wordplay.
'comeback of' says the letters should be written backwards.
'star' becomes 'sun' (Sun is an example**).
'sun'+'is'='sunis'
'sunis' in reverse letter order is 'SINUS'.
(Other definitions for sinus that I've seen before include "Cavity in body (connecting to nose)" , "Air-filled cavity of the skull" , "chamber" , "Cavity in skull connecting with nostrils" , "Anatomical cavity" .)