Skirt on back of horizontal shelf (5)
I believe the answer is:
ledge
'shelf' is the definition.
('ledge' can be a synonym of 'shelf')
'skirt on back of horizontal' is the wordplay.
'skirt' becomes 'edge' (I've seen this before**).
'on' means one lot of letters go next to another.
'back of' indicates one should take the final letters.
The final letter of 'horizontal' is 'l'.
'edge' put after 'l' is 'LEDGE'.
(Other definitions for ledge that I've seen before include "Projecting ridge on a mountain" , "Narrow projection or shelf" , "A step" , "Submerged underwater shelf" , "Surface under e.g. window" .)