In the absence of authority, some seizing chief (7)
I believe the answer is:
anarchy
'in the absence of authority' is the definition.
(anarchy is the absence of government or authority)
'some seizing chief' is the wordplay.
'some' becomes 'any' (similar in meaning).
'seizing' means one lot of letters goes inside another.
'chief' becomes 'arch' (synonyms).
'any' going around 'arch' is 'ANARCHY'.
(Other definitions for anarchy that I've seen before include "State of disorder in society" , "Unrule" , "State of lawlessness and disorder" , "Complete absence of law" , "State of civil disorder" .)