Jock's counterpart (4)
I believe the answer is:
nerd
'jock's counterpart' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for nerd that I've seen before include "Someone boring (about computers?)" , "one that's overly cerebral" , "Socially inept but knowledgeable person" , "Obsessive," , "Flag on poles; wide (headline)" .)