Left on the side of a shelf (5)
I believe the answer is:
ledge
'a shelf' is the definition.
('ledge' can be a synonym of 'shelf')
'left on the side' is the wordplay.
'left' becomes 'l' (common abbreviation).
'on' means one lot of letters go next to another.
'the side' becomes 'edge' (edge is a kind of side**).
'l'+'edge'='LEDGE'
'of' acts as a link.
(Other definitions for ledge that I've seen before include "Shelf-like projection" , "Ability to contain one's emotions" , "Narrow projection or shelf" , "found halfway up a cliff?" , "Projecting ridge on a mountain" .)