Examine old reject (4)
I believe the answer is:
veto
'reject' is the definition.
(I know that veto is a kind of disown)
'examine old' is the wordplay.
'examine' becomes 'vet' (I've seen this before).
'old' becomes 'o' (abbreviation).
'vet'+'o'='VETO'
(Other definitions for veto that I've seen before include "The power to prohibit or reject a proposal" , "Forbid; prohibition" , "Red light" , "Reject - ban" , "Power to reject legislation" .)