Thoroughly inspect old block (4)
I believe the answer is:
veto
'block' is the definition.
(to veto is to block a decision or action)
'thoroughly inspect old' is the wordplay.
'thoroughly inspect' becomes 'vet' (as in vetting someone for security purposes).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'vet'+'o'='VETO'
(Other definitions for veto that I've seen before include "Refusal to assent" , "Formally forbid" , "Rule out" , "The power to prohibit or reject a proposal" , "Reject - ban" .)