Corrosive agent mostly leading to spasmodic twitching (7)
I believe the answer is:
caustic
'corrosive' is the definition.
(I know that corrosive can be written as caustic)
'agent mostly leading to spasmodic twitching' is the wordplay.
'agent' becomes 'cause' (agent can mean the cause of something).
'mostly' means to remove the last letter.
'leading to' says to put letters next to each other (some letters go before or lead others).
'spasmodic twitching' becomes 'tic' (an involuntary twitch).
'cause' with its final letter taken off is 'caus'.
'caus'+'tic'='CAUSTIC'
(Other definitions for caustic that I've seen before include "Corrosive - cutting" , "Harsh or corrosive" , "Acrimonious - sarcastic" , "Chemically burning" , "Able to burn - cutting" .)