King is monarch having succeeded a German emperor (6)
I believe the answer is:
kaiser
'german emperor' is the definition.
(I've seen this before)
'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 present in the answer.
'is' is present in the answer.
'monarch' could be 'k' (chess abbreviation) and 'k' is found within the answer.
'a' is found in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
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" .)