Partly fills in useless hole in the head (5)
I believe the answer is:
sinus
'hole in the head' is the definition.
(I've seen this before)
'partly fills in useless' is the wordplay.
'partly' indicates the answer is hidden within the clue.
'SINUS' can be found hidden inside 'fills in useless'.
(Other definitions for sinus that I've seen before include "Cavity linked to nose" , "Hole in the head" , "snotty enclosure" , "Air-filled cavity in one's head" , "Cavity in body (connecting to nose)" .)