Copper having youthful exterior essentially swearing in native tongue (10)
I believe the answer is:
vernacular
'native tongue' is the definition.
'copper having youthful exterior essentially swearing' is the wordplay.
I cannot quite see how this works, but
'copper' could be 'cu' (chemical symbol) and 'cu' is found in the answer.
'youthful' could be 'vernal' ('vernal' can be a synonym of 'youthful') and 'vernal' is found in the remaining letters.
The remaining letters 'ar' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'in' is the link.
(Other definitions for vernacular that I've seen before include "Characteristic language" , "Local-style" , "Everyday language" , "Native tongue - dialect" , "Native language" .)