Front line? (6)
I believe the answer is:
isobar
'front 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 "Weather-map line" , "One showing pressure" , "Line on map connecting places of equal atmospheric pressure" , "Line joining places of equal barometric pressure" , "feature on some maps?" .)