Left on the side shelf (5)
I believe the answer is:
ledge
'shelf' is the definition.
(I know that shelf can be written as ledge)
'left on the side' is the wordplay.
'left' becomes 'l' (common abbreviation).
'on' says to put letters next to each other.
'the side' becomes 'edge' (edge is a kind of side).
'l'+'edge'='LEDGE'
(Other definitions for ledge that I've seen before include "Narrow shelf, often attached to window" , "Narrow projecting shelf as from window" , "rocky situation?" , "Ridge or shelf" , "A step" .)