Carry out checks on oxygen bar (4)
I believe the answer is:
veto
'bar' is the definition.
(I've seen this before)
'carry out checks on oxygen' is the wordplay.
'carry out checks on' becomes 'vet' (to vet someone is to perform security checks).
'oxygen' becomes 'o' (atomic symbol for oxygen).
'vet'+'o'='VETO'
(Other definitions for veto that I've seen before include "A confused vote, altogether negative" , "Right of rejection" , "An overriding negative vote" , "The right to forbid an action" , "Refusal to assent" .)