Where they teach only ten letters of the alphabet? (4)
I believe the answer is:
eton
'where they teach only' is the definition.
The definition suggests a singular noun which matches the answer.
'ten letters of the alphabet?' is the wordplay.
I cannot really see how this works, but
'of' could be 'o' and 'o' is present in the answer.
an anagram of 'ten' is 'etn' which is present in the left over letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for eton that I've seen before include "Note sent up to English public school" , "Public school near Windsor" , "Posh English school of note" , "Berkshire public school" , "Public school founded by Henry VI" .)