Nerd starts to acquire new obsession: rare African knives (6)
I believe the answer is:
anorak
'nerd' is the definition.
'starts to acquire new obsession rare african knives' is the wordplay.
'starts to' suggests taking the first letters.
The first letters of 'acquire new obsession rare african knives' is 'ANORAK'.
(Other definitions for anorak that I've seen before include "geek" , "good on most figures?" , "Garment (worn by boring obsessive?)" , "Arctic jacket: nerd" , "Weather-resistant fabric" .)