Form of oxygen, oxygen absent in space (4)
I believe the answer is:
zone
'space' is the definition.
(thesaurus)
'form of oxygen oxygen absent' is the wordplay.
'form of oxygen' becomes 'ozone' (I've seen this in another clue).
'oxygen' becomes 'O' (atomic symbol for oxygen).
'absent' suggests deleting specific letters.
'ozone' with 'o' taken out is 'ZONE'.
'in' acts as a link.
(Other definitions for zone that I've seen before include "Area, division" , "District, area" , "Area subject to restrictions, parking say" , "Region with particular characteristics" , "Area with a particular characteristic" .)