Sarcastic about American movement that's not voluntary (7)
I believe the answer is:
caustic
'sarcastic' is the definition.
(thesaurus)
'about american movement that's not voluntary' is the wordplay.
'about' becomes 'ca' (abbreviation for 'circa').
'american' becomes 'us' (abbreviation for United States).
'movement that's not voluntary' becomes 'tic' (involuntary bodily movement).
'ca'+'us'+'tic'='CAUSTIC'
(Other definitions for caustic that I've seen before include "Chemically burning" , "Acrid" , "Severe" , "Corrosive - cutting" , "Burning, corroding" .)