Left side shelf (5)
I believe the answer is:
ledge
'shelf' is the definition.
(I know that shelf can be written as ledge)
'left side' is the wordplay.
'left' becomes 'l' (common abbreviation).
'side' becomes 'edge' (edge is a kind of side).
'l'+'edge'='LEDGE'
(Other definitions for ledge that I've seen before include "Narrow projection or shelf" , "A step" , "Ability to contain one's emotions" , "Narrow projecting shelf as from window" , "Projecting shelf, horizontal projection" .)