Stray inside border region (3)
I believe the answer is:
err
'stray' is the definition.
('err' can be a synonym of 'stray')
'inside border region' is the wordplay.
'inside' indicates the answer is hidden within the clue.
'ERR' is hidden within 'border region'.
(Other definitions for err that I've seen before include "Stray from the straight and narrow" , "Be mistaken" , "'to... is human,to forgive divine'" , "Blunder" , "Leave the straight and narrow" .)