Extreme nerd — one might pick you up on grammar at first, I'm afraid! (8)
I believe the answer is:
ubergeek
'extreme nerd' is the definition.
(very nerdy person)
'one might pick you up on grammar at first i'm afraid' is the wordplay.
'one might pick you up' becomes 'Uber' (Ubers pick passengers up).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'at first' indicates taking the first letters.
'i'm afraid' becomes 'eek' (expression of fear).
The initial letter of 'grammar' is 'g'.
'uber'+'g'+'eek'='UBERGEEK'
(Other definitions for ubergeek that I've seen before include "extreme enthusiast" , "Nerdy student" .)