One's personal space (5)
I believe the answer is:
sinus
This might be a double definition.
'one's personal' is the first definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'space' is the second definition.
(I have seen 'Hollow space' mean 'sinus' so perhaps 'space' could also mean 'sinus')
Can you help me to learn more?
(Other definitions for sinus that I've seen before include "Bodily cavity" , "Cavity within a bone" , "Cavity in the facial bones" , "Head space" , "Air-filled cavity of the skull" .)