A collection of letters (8)
I believe the answer is:
alphabet
'letters' is the definition.
(I've seen this before)
'a collection' is the wordplay.
'a' becomes 'alpha' (the Greek letter A).
'collection' becomes 'bet' (I am not sure about this - if you are sure you should believe this answer much more).
'alpha'+'bet'='ALPHABET'
'of' is the link.
Can you help me to learn more?
(Other definitions for alphabet that I've seen before include "A to Z inclusive" , "Pale bath (anag.)" , "Set of characters" , "All the characters together" , "Ordered set of letters" .)