Starting to snort cocaine, one with a nervous twitch and a nervous complaint (8)
I believe the answer is:
sciatica
'a nervous complaint' is the definition.
(medical complaint caused by an irritated nerve)
'starting to snort cocaine one with a nervous twitch' is the wordplay.
'starting to' suggests taking the first letters.
'cocaine one' becomes 'cia' (I am not sure about this - if you are sure you should believe this answer much more).
'with' is an insertion indicator.
'nervous twitch' becomes 'tic' (involuntary twitch in the body).
The first letter of 'snort' is 's'.
'a'+'tic'='atic'
'cia' going around 'atic' is 'ciatica'.
's'+'ciatica'='SCIATICA'
'and' is the link.
(Other definitions for sciatica that I've seen before include "Compression of nerve roots in back and thigh" , "Pain in the back, hip and leg" , "Lower back and hip pain" , "Pain around back or hip" , "Intense pain in the leg" .)