Horizontal invention (5)
I believe the answer is:
lying
This clue might be a double definition.
'horizontal' is the first definition.
Both the answer and definition are adjectives. Perhaps there's a link between them I don't understand?
'invention' is the second definition.
(I've seen this before)
Can you help me to learn more?
(Other definitions for lying that I've seen before include "Deviating from truth" , "Anything but upright" , "Not being disturbed" , "Spread out" , "Duplicity" .)