Original ally crossing boundary (6)
I believe the answer is:
primal
'original' is the definition.
(synonyms)
'ally crossing boundary' is the wordplay.
'ally' becomes 'pal' (both can mean a friend or associate).
'crossing' indicates putting letters inside.
'boundary' becomes 'rim' (rim is a kind of boundary).
'pal' enclosing 'rim' is 'PRIMAL'.
(Other definitions for primal that I've seen before include "Original - basic" , "Sort of scream" , "Primeval, original" , "Having existed from the beginning" , "typifying some fear?" .)