Brainy sort (4)
I believe the answer is:
nerd
'brainy sort' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for nerd that I've seen before include "Sort of chap" , "Feeble person (sl.)" , "Boringly studious person" , "In slang, a foolish or uninteresting person" , "Foolish or annoying person" .)