Imperial title periodically at issue between two kings (6)
I believe the answer is:
kaiser
'imperial title' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'periodically at issue between two kings' is the wordplay.
'two kings' means two substitutions for 'king'.
'periodically' means one should take alternating letters (letters must be selected 'every so often').
'between' indicates putting letters inside.
'king' becomes 'k' (abbreviation for king in chess).
'king' becomes 'R' (abbreviation of Latin rex).
'at'+'issue'='atissue'
The alternating letters of 'atissue' are 'aise'.
'k'+'r'='kr'
'aise' put into 'kr' is 'KAISER'.
Can you help me to learn more?
(Other definitions for kaiser that I've seen before include "Old German emperor like Bill" , "Austrian bigwig" , "Old German emperor, pre-Hitler" , "Former German emperor" , "Emperor of Austria or Germany until 1918" .)