Shelf left at border (5)
I believe the answer is:
ledge
'shelf' is the definition.
(I know that shelf can be written as ledge)
'left at border' is the wordplay.
'left' becomes 'l' (common abbreviation).
'at' means one lot of letters go next to another.
'border' becomes 'edge' (synonyms).
'l'+'edge'='LEDGE'
(Other definitions for ledge that I've seen before include "Narrow shelf, often attached to window" , "rocky situation?" , "found halfway up a cliff?" , "Projecting shelf, horizontal projection" , "Projecting ridge on a mountain" .)