Line in an article Boris is upset about (6)
I believe the answer is:
isobar
'line' is the definition.
(I know that isobar is a type of isarithm)
'an article boris is upset about' is the wordplay.
'an article' becomes 'a' (the indefinite article in English).
'is upset' is an anagram indicator (I've seen 'are upset' mean this).
'about' means one lot of letters goes inside another.
'boris' is an anagram of 'isobr'.
'a' inserted inside 'isobr' is 'ISOBAR'.
'in' is the link.
(Other definitions for isobar that I've seen before include "Line on map indicating atmospheric pressure" , "One showing pressure" , "It appears on some maps" , "Air pressure line on a weather chart" , "Pressure line on a weather map" .)