Mix up in heart of Brussels, nothing concerning but a pain in the backside (6,4)
I believe the answer is:
saddle sore
'a pain in the backside' is the definition.
I can't judge whether this defines the answer.
'mix up in heart of brussels nothing concerning' is the wordplay.
'mix up' becomes 'addle' (addling is a kind of mixing up).
'in' means one lot of letters goes inside another.
'heart of' says to take the centre.
'nothing' becomes 'O' (looks like zero - 0).
'concerning' becomes 're' (both can mean on a particular topic).
The centre of 'brussels' is 'ss'.
'addle' going inside 'ss' is 'saddles'.
'saddles'+'o'+'re'='SADDLE-SORE'
'but' acts as a link.
Can you help me to learn more?
(Other definitions for saddle sore that I've seen before include "Uncomfortable when up" , "Chafed by riding" .)