Son of Darius succeeded old king from the East (6)
I believe the answer is:
xerxes
'son of darius' is the definition.
The definition suggests a singular noun which matches the answer.
'succeeded old king from the east' is the wordplay.
'succeeded' becomes 's' (genealogical abbreviation).
'old' becomes 'ex'.
'king' becomes 'rex' (I've seen this before).
'from the east' says the letters should be written in reverse.
's'+'ex'+'rex'='sexrex'
'sexrex' reversed gives 'XERXES'.
Can you help me to learn more?
(Other definitions for xerxes that I've seen before include "King of Persia, son of Darius" , "------I, son of Darius I, attempted invader of Greece, 480-79 BC" , "Persian king, son of Darius (bridge of boats)" , "One old king" , "Name of two Persian kings" .)