He joins a hundred twitching and feverish (6)
I believe the answer is:
hectic
'feverish' is the definition.
(I know that feverish can be written as hectic)
'he joins a hundred twitching' is the wordplay.
'joins' says to put letters next to each other.
'a hundred' becomes 'C' (Roman numerals).
'twitching' becomes 'tic' (tic is a kind of twitching).
'he'+'c'+'tic'='HECTIC'
'and' is the link.
(Other definitions for hectic that I've seen before include "Frantically active" , "'Excited, flustered or rushed (6)'" , "Frantically busy" , "Marked by intense agitation - fevered" , "'Extremely busy, rushed (6)'" .)