First mate crossing border (6)
I believe the answer is:
primal
'first' is the definition.
(primal can mean early or first)
'mate crossing border' is the wordplay.
'mate' becomes 'pal' (I've seen this before).
'crossing' indicates putting letters inside (to cross can mean to go over or around).
'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; chief" , "Original, fundamental" , "Primitive or fundamental" , "Having existed from the beginning" , "From a very early time" .)