Month and year in which to take on one kind of state (8)
I believe the answer is:
monarchy
'state' is the definition.
(I have seen 'Sovereign state ' mean 'monarchy' so perhaps 'state' could also mean 'monarchy')
'month and year in which to take on one kind' is the wordplay.
'month' becomes 'mo' (abbreviation for month).
'and' becomes 'n' (common abbreviation for 'and').
'year' becomes 'y'.
'in which' indicates putting letters inside.
'to take on one kind' becomes 'arch' (I am not sure about this - if you are sure you should believe this answer much more).
'n'+'y'='ny'
'ny' going around 'arch' is 'narchy'.
'mo'+'narchy'='MONARCHY'
'of' is the link.
Can you help me to learn more?
(Other definitions for monarchy that I've seen before include "realm" , "Kingdom - kingship" , "Kingship system" , "Sovereign state" , "Rule by a king or queen" .)