The meteorologist's line (6)
I believe the answer is:
isobar
'the meteorologist's line' is the definition.
(I've seen this in another clue)
'meteorologist's line' is the wordplay.
'meteorologist' becomes 'iso' (I can't explain this - if you can you should believe this answer much more).
'line' becomes 'bar line' (bar line is a kind of line).
'iso'+'bar'='ISOBAR'
Can you help me to learn more?
(Other definitions for isobar that I've seen before include "Line on map connecting places of equal atmospheric pressure" , "Line showing points having equal barometric pressure at a given time" , "Air pressure line on a weather chart" , "It appears on some maps" , "Weather-map line" .)