Line on map, one originally standing on counter (6)
I believe the answer is:
isobar
'line on map' is the definition.
(line indicating pressure on a map)
'one originally standing on counter' is the wordplay.
'one' becomes 'I' (Roman numeral).
'originally' indicates taking the first letters.
'counter' becomes 'bar' (bar is a kind of counter).
The first letters of 'standing on' is 'so'.
'i'+'so'+'bar'='ISOBAR'
(Other definitions for isobar that I've seen before include "meteorologist's line" , "Meteorological line joining points of equal pressure" , "Line showing points having equal barometric pressure at a given time" , "What's shown on some maps" , "Line of equal barometric pressure on a map" .)