Old overworked horse? Yes, half dead (4)
I believe the answer is:
jade
'old overworked horse?' is the definition.
I can't tell whether this definition defines the answer.
'yes half dead' is the wordplay.
'yes' becomes 'ja' (I can't justify this - if you can you should give a lot more credence to this answer).
'half' suggests halving the letters.
'dead' halved is 'de'.
'ja'+'de'='JADE'
Can you help me to learn more?
(Other definitions for jade that I've seen before include "Semiprecious stone - shade of green" , "disreputable woman" , "Mount" , "Semi-precious gemstone -- tired old horse" , "Green gem" .)