Countermand second set of changes? (6)
I believe the answer is:
repeal
'countermand' is the definition.
(synonyms)
'second set of changes?' is the wordplay.
'second' becomes 're' (prefix meaning second in eg review, re-education).
'set of changes?' becomes 'peal' (a peal is a set of changes rung on bells).
're'+'peal'='REPEAL'
(Other definitions for repeal that I've seen before include "Cancel or nullify a law" , "Rescind or withdraw a law" , "'Annul, revoke (6)'" , "Cancel officially, annul" , "Withdraw law officially" .)