He was a so-called king (5)
I believe the answer is:
rufus
This clue might be a double definition.
'he' is the first definition.
'a so-called king' is the second definition.
'rufus' can be an answer for 'king' (I've seen this before). I am not sure about the 'a so-called' bit.
'was' acts as a link.
Can you help me to learn more?
(Other definitions for rufus that I've seen before include "one killed in New Forest" , "Boy" , "redhead" , "King William II nickname" , "Red-headed man possibly" .)