Jerks back clot in message (6)
I believe the answer is:
spasms
'jerks' is the definition.
(I've seen this in another clue)
'back clot in message' is the wordplay.
'back' shows that the letters should be reversed in order.
'clot' becomes 'sap' (both can mean an idiot).
'in' means one lot of letters goes inside another.
'message' becomes 'SMS' (Short Message Service, text messaging).
'sap' going within 'sms' is 'smsaps'.
'smsaps' reversed gives 'SPASMS'.
(Other definitions for spasms that I've seen before include "Seizures and convulsions" , "sudden bursts of activity" , "Convulsions or cramps" , "Nervous twitches and painful convulsions" , "Convulsions, fits" .)