Speck on outside of large jar (4)
I believe the answer is:
jolt
'jar' is the definition.
('jolt' can be a synonym of 'jar')
'speck on outside of large' is the wordplay.
'speck' becomes 'jot' ('jot' can be a synonym of 'speck'**).
'on outside of' is an insertion indicator.
'large' becomes 'l'.
'jot' enclosing 'l' is 'JOLT'.
(Other definitions for jolt that I've seen before include "elbow" , "Sudden jerk" , "Sudden shock" , "Jostle; shock" , "bump" .)