The desire to upset socially superior Reg (4)
I believe the answer is:
urge
'the desire' is the definition.
(I've seen this in other clues)
'upset socially superior reg' is the wordplay.
I cannot quite understand how this works, but
'superior' could be 'u' (U can mean upper-class, as in U and non-U speech) and 'u' is located in the answer.
an anagram of 'reg' is 'rge' which is present in the answer.
No letters remain.
This explanation may well be incorrect...
'to' is the link.
Can you help me to learn more?
(Other definitions for urge that I've seen before include "Implore" , "Advise earnestly, spur" , "Advise strongly or prod" , "Press to do something" , "Impel - impulse" .)