Boring student starts to name each ruddy dog! (4)
'boring student' is the definition.
(term for a boringly studious person)
'starts to name each ruddy dog' is the wordplay.
'starts to' says to take the initial letters.
The initial letters of 'name each ruddy dog' is 'NERD'.
(Other definitions for nerd that I've seen before include "One lacking social skills", "Sort of chap", "Irritating person (slang)", "Anorak (informal)", "Technical expert".)