Newly fitted up pad with recliner -- in which one won't be this! (13)
I believe the answer is:
perpendicular
'newly fitted up pad with recliner in which one won't be this' is the definition.
The definition suggests a singular noun which matches the answer.
'one won't be this' is the wordplay.
I cannot really see how this works, but
'one' could be 'i' (Roman numeral) and 'i' is found in the answer.
'be' could be 'end' (ending is a kind of being) and 'end' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for perpendicular that I've seen before include "style of construction" , "Like this answer [1A], relative to 1 dn" , "Vertical - at 90 degrees" , "Vertical; upright" , "Style of late English Gothic church architecture" .)