He shows that one letter's not the whole alphabet (5)
I believe the answer is:
ralph
'he' is the definition.
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for ralph that I've seen before include "N Waldo Emerson, nineteenth century US poet" , "Chap" , "___ Fiennes, English actor, b. 1962" , "Boy's name" , "Fellow's name" .)