Joy's European ox that is nicked regularly (10)
I believe the answer is:
ebullience
'joy's' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'european ox that is nicked regularly' is the wordplay.
'european' becomes 'E' (abbreviation e.g. EU).
'ox' becomes 'bull' (I can't justify this - if you can you should give a lot more credence to this answer).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternating letters of 'nicked' are 'nce'.
'e'+'bull'+'ie'+'nce'='EBULLIENCE'
Can you help me to learn more?
(Other definitions for ebullience that I've seen before include "Exuberance" , "Enthusiastic excitement" , "Cheerful enthusiasm; boiling over" .)