Took on title, as king abdicated initially (7,4)
I believe the answer is:
assumed name
'king abdicated initially' is the definition.
I can't tell whether this definition defines the answer.
'took on title' is the wordplay.
'took on' becomes 'assumed' ('assume' can be a synonym of 'take on').
'title' becomes 'name' (I've seen this before).
'assumed'+'name'='ASSUMED NAME'
'as' acts as a link.
Can you help me to learn more?
(Other definitions for assumed name that I've seen before include "I may appear under this" , "Alias" .)