Old king or noted military leader (4)
I believe the answer is:
cole
'old king' is the definition.
(nursery rhyme character Old King Cole)
'noted military leader' is the wordplay.
'noted military' becomes 'col' (I can't justify this - if you can you should give a lot more credence to this answer).
'leader' becomes 'e' (I've seen this in other clues).
'col'+'e'='COLE'
'or' acts as a link.
Can you help me to learn more?
(Other definitions for cole that I've seen before include "Old nursery rhyme king" , "Cabbage" , "Old English king!" , "died in 2015" , "Elderly monarch" .)