King is monarch having succeeded a German emperor (6)
I believe the answer is:
kaiser
'german emperor' is the definition.
(I've seen this in another clue)
'king is monarch having succeeded a' is the wordplay.
I cannot really see how this works, but
'king' could be 'er' (abbreviation for Edward Rex, various English kings) and 'er' is found within the answer.
'is' is present in the answer.
'monarch' could be 'k' (chess abbreviation) and 'k' is present in the answer.
'a' is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for kaiser that I've seen before include "Former German emperor" , "Old German emperor like Bill" , "Austrian bigwig" , "Emperor of Austria or Germany until 1918" , "Ex-ruler of Germany" .)