King of Northumbria emerged drained by victory (5)
I believe the answer is:
edwin
'king of northumbria' is the definition.
(I've seen this before)
'emerged drained by victory' is the wordplay.
'drained' means to remove the middle letters.
'by' is a charade indicator (letters next to each other).
'victory' becomes 'win' (a win is a victory).
'emerged' with its middle taken out is 'ed'.
'ed'+'win'='EDWIN'
(Other definitions for edwin that I've seen before include "C7 king of Northumbria" , "Anglo-Saxon King" , "old king" , "ancient king" , "-- Hubble (astronomer)" .)