Specialist language almost disappeared following shake up (6)
I believe the answer is:
jargon
'specialist language' is the definition.
(I've seen this before)
'almost disappeared following shake up' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'disappeared' becomes 'gone' (going is a kind of disappearing).
'following' means one lot of letters go next to another.
'shake up' becomes 'jar' ('jar' can be a synonym of 'shake up'**).
'gone' with its final letter taken away is 'gon'.
'gon' put after 'jar' is 'JARGON'.
(Other definitions for jargon that I've seen before include "Bandied words, possibly" , "Language specific to an activity" , "Specialised language" , "Words and expressions used within a particular group" , "Technical language" .)