Nerd's short expression of surprise (3)
I believe the answer is:
gee
'expression of surprise' is the definition.
(I've seen this before)
'nerd's short' is the wordplay.
'nerd' becomes 'geek' (I've seen this before).
'short' means to remove the last letter.
'geek' with its last letter removed is 'GEE'.
(Other definitions for gee that I've seen before include "Wow!" , "- - whiz! (US)" , "Gracious" , "Exclamation of surprise" , "Urge horse - expression of surprise" .)