One gives a teacher more fillers (5)
I believe the answer is:
donor
'one' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'a teacher more fillers' is the wordplay.
'a teacher' becomes 'don' (don is a kind of teacher).
'more fillers' becomes 'or' (I am not sure about this - if you are sure you should believe this answer much more).
'don'+'or'='DONOR'
'gives' acts as a link.
Can you help me to learn more?
(Other definitions for donor that I've seen before include "A giver, often of blood" , "Philanthropist" , "Charitable giver" , "Giver, of blood say" , "Someone providing" .)