Shock line in note (4)
I believe the answer is:
jolt
'shock' is the definition.
(thesaurus)
'line in note' is the wordplay.
'line' becomes 'l'.
'in' means one lot of letters goes inside another.
'note' becomes 'jot' (jot is a kind of note**).
'l' put inside 'jot' is 'JOLT'.
(Other definitions for jolt that I've seen before include "Sudden jerk" , "elbow" , "A sudden jar" , "bump" , "Sudden impact" .)