Son opening unwanted mail makes sudden attack (5)
I believe the answer is:
spasm
'sudden attack' is the definition.
(I've seen this before)
'son opening unwanted mail' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'opening' indicates putting letters inside (some letters open a gap in others).
'unwanted mail' becomes 'sp am' (I've seen this in another clue).
's' placed inside 'spam' is 'SPASM'.
'makes' acts as a link.
(Other definitions for spasm that I've seen before include "Painful muscle contraction" , "Tic, sudden contraction" , "Fit" , "Sudden convulsion" , "Muscular convulsion" .)