Nevertheless, follows American border (4)
I believe the answer is:
abut
'border' is the definition.
(synonyms)
'nevertheless follows american' is the wordplay.
'nevertheless' becomes 'but' (I've seen this in another clue).
'follows' says to put letters next to each other.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'but' after 'a' is 'ABUT'.
(Other definitions for abut that I've seen before include "Impinge" , "Touch, lean on" , "Adjoin, press up against" , "Be next to -- share a boundary" , "Of building or land, end or lean against" .)