Son opening unwanted mail in sudden attack (5)
I believe the answer is:
spasm
'sudden attack' is the definition.
(I've seen this before)
'son opening unwanted mail in' is the wordplay.
'opening' indicates taking the first letters.
'unwanted mail' becomes 'spam' (I've seen this before).
'in' means one lot of letters goes inside another.
The initial letter of 'son' is 's'.
's' put into 'spam' is 'SPASM'.
(Other definitions for spasm that I've seen before include "Jerk" , "sudden attack" , "Involuntary muscular movement" , "Convulsive action" , "Sudden involuntary contraction of a muscle" .)