Maize ration for an eager beaver (6)
I believe the answer is:
zealot
'eager beaver' is the definition.
(thesaurus)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for zealot that I've seen before include "Fervent enthusiast" , "Fanatic for a cause" , "Fanatic, obsessed person" , "Fervent, even militant, proponent of something" , "Single-minded supporter of a cause" .)