Nerd or his jacket (6)
I believe the answer is:
anorak
This clue might be a double definition.
'nerd' is the first definition.
'his jacket' is the second definition.
'anorak' can be an answer for 'jacket' (anorak is a kind of jacket). I am unsure of the 'his' bit.
'or' acts as a link.
Can you help me to learn more?
(Other definitions for anorak that I've seen before include "Coat (for the boringly obsessive?)" , "trainspotter" , "Trivia obsessive" , "Casual outerwear" , "Zipped jacket with hood attached" .)