Mocking duke leaving this country on horseback, perhaps (8)
I believe the answer is:
deriding
'mocking' is the definition.
(deriding is a kind of mocking)
'duke leaving this country on horseback perhaps' is the wordplay.
'leaving' suggests deleting specific letters.
'this country' becomes 'UK' (United Kingdom).
'on horseback perhaps' becomes 'riding' (I can't justify this - if you can you should give a lot more credence to this answer).
'duke' with 'uk' removed is 'de'.
'de'+'riding'='DERIDING'
Can you help me to learn more?
(Other definitions for deriding that I've seen before include "Ridiculing" , "Mocking" .)