Letters and numbers for letters! (8)
I believe the answer is:
postcode
'letters' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'letters and numbers' is the wordplay.
'letters' becomes 'post' (letters in the mail or post).
'and' says to put letters next to each other.
'numbers' becomes 'code' (I can't justify this - if you can you should give a lot more credence to this answer).
'post'+'code'='POSTCODE'
'for' is the link.
Can you help me to learn more?
(Other definitions for postcode that I've seen before include "Conclusion of address" , "Indication of area in address" , "End of address" , "Eg, E-Nine-Eight One-T-T" , "Letter sorting device (or kind of modern lottery?)" .)