Pressure lines (7)
I believe the answer is:
isobars
'pressure lines' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for isobars that I've seen before include "Lines of equal atmospheric pressure" , "can show what pressure folk are under" , "Air boss (anag.)" , "What may be featured on map" , "Lines on a weather map" .)