Perverse desire to inter upper-class remains (7)
I believe the answer is:
residue
'remains' is the definition.
(thesaurus)
'perverse desire to inter upper-class' is the wordplay.
I cannot really understand how this works, but
'upper-class' could be 'u' ('u' can be similar in meaning to 'upper-class') and 'u' is located in the answer.
an anagram of 'desire' is 'reside' which is present in the left over letters.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for residue that I've seen before include "Stuff left over" , "What is left over, remainder (7)" , "grounds" , "What's left" , "Part remaining" .)