Language used by violent youth ultimately on main road (4)
I believe the answer is:
thai
'language' is the definition.
(Thai is an example)
'violent youth ultimately on main road' is the wordplay.
'ultimately' suggests the final letters.
'on' means one lot of letters go next to another.
'main road' becomes 'ai' (resembles A1, major UK road).
The last letters of 'violent youth' are 'th'.
'th'+'ai'='THAI'
'used by' is the link.
(Other definitions for thai that I've seen before include "An Asian cuisine" , "From Bangkok, for example" , "is concerned with an eastern country" , "Asian tongue" , "A hit (anag)" .)