Line in short note is a shock (4)
I believe the answer is:
jolt
'shock' is the definition.
(I know that shock can be written as jolt)
'line in short note' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'in' indicates putting letters inside.
'short note' becomes 'jot' (jot is a kind of note. I am not sure about the 'short' bit.).
'l' placed within 'jot' is 'JOLT'.
'is a' is the link.
Can you help me to learn more?
(Other definitions for jolt that I've seen before include "bump" , "elbow" , "Sudden impact" , "Shock, jerk" , "Jostle; shock" .)