Original ally seen to cross border (6)
I believe the answer is:
primal
'original' is the definition.
(synonyms)
'ally seen to cross border' is the wordplay.
'ally' becomes 'pal' (both can mean a friend or associate).
'seen to cross' indicates putting letters inside (some letters cross over others).
'border' becomes 'rim' (both can mean an edge).
'pal' placed around 'rim' is 'PRIMAL'.
(Other definitions for primal that I've seen before include "Original, fundamental" , "Original - basic" , "Relating to an early stage of evolution" , "Sort of scream" , "From a very early time" .)