Early barely understood slang (5)
I believe the answer is:
argot
'slang' is the definition.
(synonyms)
'early barely understood' is the wordplay.
'early' becomes 'rare' (I've seen this before).
'barely' means to look at the middle letters (the word is bare of its external letters).
'understood' becomes 'got' (getting is a kind of understanding).
The middle letters of 'rare' are 'ar'.
'ar'+'got'='ARGOT'
(Other definitions for argot that I've seen before include "manner of speaking" , "Particular group's jargon" , "Specialised language" , "Special jargon of a group" , "Cant" .)