Pressure line on a map (6)
I believe the answer is:
isobar
'pressure line on a map' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for isobar that I've seen before include "Line of equal barometric pressure on a map" , "Meteorological line joining points of equal pressure" , "Weather-map line" , "Line showing points having equal barometric pressure at a given time" , "Contour" .)