It's opposed to both Windsor and Harrow (4)
I believe the answer is:
eton
'harrow' is the definition.
(I've seen this in another clue)
'it's opposed to both windsor and' is the wordplay.
I cannot really see how this works, but
'to' is present in the answer.
'and' could be 'n' (common abbreviation for 'and') and 'n' is found in the answer.
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is found in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for eton that I've seen before include "College in Berkshire" , "School near Slough" , "Place for learning" , "Leading English public school" , "Famous English public school" .)