Maybe Stalin's rule hit Castro badly, going into decline (12)
I believe the answer is:
dictatorship
'maybe stalin's rule' is the definition.
I can't judge whether this definition defines the answer.
'hit castro badly going into decline' is the wordplay.
'badly' is an anagram indicator (letters in the wrong order).
'going into' means one lot of letters goes inside another.
'decline' becomes 'dip' (both can mean to fall or droop).
'hit'+'castro'='hitcastro'
'hitcastro' anagrammed gives 'ctatorshi'.
'ctatorshi' going within 'dip' is 'DICTATORSHIP'.
Can you help me to learn more?
(Other definitions for dictatorship that I've seen before include "Absolute form of government" , "Absolute rule" , "Totalitarian regime" , "Government by such as Julius Caesar" , "Form of rule" .)