Left on side of shelf (5)
I believe the answer is:
ledge
'shelf' is the definition.
(synonyms)
'left on side' is the wordplay.
'left' becomes 'l' (common abbreviation).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'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 "Narrow shelf, often attached to window" , "Shelf-like projection" , "Projecting shelf, horizontal projection" , "Ridge or shelf" , "A step" .)