To GB, it is Europe that appears horribly middle-class (5,9)
I believe the answer is:
petit bourgeois
'class' is the definition.
(petit bourgeois is a kind of class)
'to gb it is europe that appears horribly' is the wordplay.
'it is' becomes 'itis'.
'that appears' means one lot of letters go next to another (some letters appear after others).
'horribly' is an anagram indicator (a horribly wrong version of the letters).
'to'+'gb'+'itis'+'europe'='togbitiseurope'
'togbitiseurope' with letters rearranged gives 'PETIT BOURGEOIS'.
'middle' is the link.
This may not be right. It may be part of another bit of the clue.
Can you help me to learn more?
(Another definition for petit bourgeois that I've seen is " Lower middle class (person)".)