Line on a map connecting points of equal atmospheric pressure (6)
I believe the answer is:
isobar
'line on a map connecting points of equal atmospheric pressure' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for isobar that I've seen before include "Map-line showing places with equal air pressure" , "Line connecting points of equal air pressure" , "(Map) line of constant atmospheric pressure" , "Line of equal barometric pressure on a map" , "Pressure line on a weather map" .)