Heartless piece produced by old military leader (6)
I believe the answer is:
shogun
'old military leader' is the definition.
'shogun' can be an answer for 'leader'. I am not certain of the 'old military' bit.
'heartless piece' is the wordplay.
'heartless' becomes 'sho' (I can't explain this - if you can you should give a lot more credence to this answer).
'piece' becomes 'gun' (I've seen this before).
'sho'+'gun'='SHOGUN'
'produced by' is the link.
Can you help me to learn more?
(Other definitions for shogun that I've seen before include "Hereditary army commander in feudal Japan" , "Japanese head" , "Hereditary military dictator of Japan until 1868" , "Former military governor of Japan" , "Hereditary Japanese military governor" .)