Meteorologist's pressure line (6)
I believe the answer is:
isobar
'meteorologist's pressure line' 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 "It indicates pressure" , "Met map pressure line" , "Contour" , "feature on some maps?" , "It appears on some maps" .)