Err, but remain outside right (5)
I believe the answer is:
stray
'err' is the definition.
('stray' can be a synonym of 'err')
'remain outside right' is the wordplay.
'remain' becomes 'stay' (staying is a kind of remaining).
'outside' indicates putting letters inside.
'right' becomes 'r' (common abbreviation).
'stay' placed around 'r' is 'STRAY'.
'but' is the link.
(Other definitions for stray that I've seen before include "Wander off course" , "Depart from the right path" , "Homeless cat" , "Lost pet" , "Wander from the path" .)