Shelf left on side (5)
I believe the answer is:
ledge
'shelf' is the definition.
(I know that shelf can be written as ledge)
'left on side' is the wordplay.
'left' becomes 'l' (common abbreviation).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'side' becomes 'edge' (edge is a kind of side).
'l'+'edge'='LEDGE'
(Other definitions for ledge that I've seen before include "Narrow projecting shelf, on cliff or building" , "rocky situation?" , "Rock shelf" , "Submerged underwater shelf" , "Ability to contain one's emotions" .)