Clash finished? Not quite and few may understand it (6)
I believe the answer is:
jargon
'few may understand it' is the definition.
(words used by only a select group)
'clash finished? not quite' is the wordplay.
'clash' becomes 'jar' (synonyms).
'finished?' becomes 'gone' (going is a kind of finishing).
'not quite' means to remove the last letter.
'gone' with its last letter removed is 'gon'.
'jar'+'gon'='JARGON'
'and' acts as a link.
(Other definitions for jargon that I've seen before include "Specialised terminology" , "special language" , "Specialised language" , "Professional terminology" , "Language specific to an activity" .)