Line of equal pressure (6)
I believe the answer is:
isobar
'line of equal pressure' is the definition.
(line indicating pressure on a map)
This is the entire clue.
(Other definitions for isobar that I've seen before include "Line showing points having equal barometric pressure at a given time" , "As brio (anag)" , "Line joining places of equal barometric pressure" , "Pressure line on a weather map" , "depression indicator?" .)