I do well to conceal black line on map (6)
I believe the answer is:
isobar
'line on map' is the definition.
(line indicating pressure on a map)
'i do well to conceal black' is the wordplay.
'do well' becomes 'soar' (soar can mean to succeed or do well).
'to conceal' is an insertion indicator.
'black' becomes 'b' (as in pencil classifications).
'soar' placed around 'b' is 'sobar'.
'i'+'sobar'='ISOBAR'
(Other definitions for isobar that I've seen before include "One showing pressure" , "Meteorological line joining points of equal pressure" , "Contour" , "Line on map connecting areas of equal pressure" , "As brio (anag)" .)