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