Original friend across the border (6)
I believe the answer is:
primal
'original' is the definition.
('primal' can be a synonym of 'original')
'friend across the border' is the wordplay.
'friend' becomes 'pal' (pal is a kind of friend).
'across' means one lot of letters goes inside another.
'the border' becomes 'rim' (I've seen this before).
'pal' placed around 'rim' is 'PRIMAL'.
(Other definitions for primal that I've seen before include "Primitive or fundamental" , "From a very early time" , "Of an early stage of development" , "Original - basic" , "typifying some fear?" .)