Cut off one’s oxygen behind time (7)
I believe the answer is:
isolate
'cut off' is the definition.
(thesaurus)
'one's oxygen behind time' is the wordplay.
'one's' becomes 'is' (Roman numeral).
'oxygen' becomes 'o' (atomic symbol for oxygen).
'behind time' becomes 'late' (I've seen this before).
'is'+'o'+'late'='ISOLATE'
(Other definitions for isolate that I've seen before include "Point out" , "Eat soil (anag)" , "Put into quarantine" , "put away?" , "Place apart, away from others" .)