Geek (4)
I believe the answer is:
'geek' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for nerd that I've seen before include "Irritating person (slang)" , "Intelligent but single-minded expert" , "Foolish or annoying person" , "Studious bore" , "Geek, anorak (sl.)" .)