Mad fellow, Frenchman, with nervous habit (8)
I believe the answer is:
frenetic
'mad' is the definition.
(thesaurus)
'fellow frenchman with nervous habit' is the wordplay.
'fellow' becomes 'f' (abbreviation - of a society etc.).
'frenchman' becomes 'rene'.
'with' means one lot of letters go next to another.
'nervous habit' becomes 'tic' (I've seen this before).
'f'+'rene'+'tic'='FRENETIC'
(Other definitions for frenetic that I've seen before include "Hectic" , "Excited, overwrought" , "Demented" , "Wildly fast and energetic" , "Distractedly hyperactive" .)