This shows pressure is zero impediment (6)
I believe the answer is:
isobar
'this shows pressure' is the definition.
The definition suggests a singular noun which matches the answer.
'is zero impediment' is the wordplay.
'zero' becomes 'O' (O resembles 0).
'impediment' becomes 'bar' (a bar impedes or blocks something).
'is'+'o'+'bar'='ISOBAR'
Can you help me to learn more?
(Other definitions for isobar that I've seen before include "It connects points of equal air pressure" , "curve on map" , "It appears on some maps" , "Map-line showing places with equal air pressure" , "Line on map connecting places of equal atmospheric pressure" .)