Check on oxygen embargo (4)
I believe the answer is:
veto
'embargo' is the definition.
(I've seen this before)
'check on oxygen' is the wordplay.
'check' becomes 'vet' (I've seen this in other clues).
'on' means one lot of letters go next to another.
'oxygen' becomes 'O' (atomic symbol for oxygen).
'vet'+'o'='VETO'
(Other definitions for veto that I've seen before include "Vote that blocks a decision" , "Vote to block a decision" , "Right to ban" , "Right of rejection" , "Stop" .)