Oppressive old emperor no longer working (7)
I believe the answer is:
onerous
'oppressive' is the definition.
(onerous can mean crushing or oppressive)
'old emperor no longer working' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'emperor' becomes 'nero' (Roman emperor).
'no longer working' becomes 'us' (I can't justify this - if you can you should give a lot more credence to this answer).
'o'+'nero'+'us'='ONEROUS'
Can you help me to learn more?
(Other definitions for onerous that I've seen before include "Burdensome, taxing" , "Heavy, difficult to do or bear" , "Challenging" , "Burdensome, oppressive" , "'Heavy, burdensome (7)'" .)