Acid set off endless spasm (7)
I believe the answer is:
caustic
'acid' is the definition.
(I know that acid can be written as caustic)
'set off endless spasm' is the wordplay.
I cannot really understand how this works, but
'spasm' could be 'tic' (tic is a kind of spasm) and 'tic' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for caustic that I've seen before include "Burning - cutting" , "Bitter - corrosive" , "Scathing" , "Acrid" , "Burning, corroding" .)