Noble old king, 50, beating a retreat (4)
I believe the answer is:
earl
'noble' is the definition.
(earl is a kind of noble)
'old king 50 beating a retreat' is the wordplay.
'old king' becomes 'ER' (abbreviation for Edward Rex, various English kings).
'50' means to take half the letters (50 percent of the letters).
'beating' becomes 'lr' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'retreat' shows that the letters should be reversed in order.
'lr'+'a'='lra'
'lra' reversed gives 'arl'.
'er' halved is 'e'.
'e'+'arl'='EARL'
Can you help me to learn more?
(Other definitions for earl that I've seen before include "UK nobleman ranking between marquess and viscount" , "Countess his wife" , "Rank below marquess" , "Male equivalent of a countess" , "British noble" .)