Hollow space? Not out visiting South America (5)
I believe the answer is:
sinus
'hollow space?' is the definition.
(I've seen this before)
'not out visiting south america' is the wordplay.
'not out' becomes 'in' ('in' is the opposite to 'out').
'visiting' means one lot of letters goes inside another.
'south' becomes 's' ('S' can be a synonym of 'south').
'america' becomes 'us' (abbreviation for the United States).
's'+'us'='sus'
'in' inserted into 'sus' is 'SINUS'.
(Other definitions for sinus that I've seen before include "Anatomical cavity" , "Head space" , "Air cavity in the cranial bones" , "Hole in the head" , "Air-filled cavity of the skull" .)