Feverish man about to have nervous twitch (6)
I believe the answer is:
hectic
'feverish' is the definition.
(I know that feverish can be written as hectic)
'man about to have nervous twitch' is the wordplay.
'man' becomes 'he' (he refers to a man).
'about' becomes 'c' (circa).
'to have' says to put letters next to each other.
'nervous twitch' becomes 'tic' (involuntary twitch in the body).
'he'+'c'+'tic'='HECTIC'
(Other definitions for hectic that I've seen before include "Frenetic" , "Full of frantic activity" , "Furious" , "Marked by intense activity" , "Involving feverish activity, flushed" .)