Boris is disturbed about a pressure line (6)
I believe the answer is:
isobar
'pressure line' is the definition.
(I've seen this before)
'boris is disturbed about a' is the wordplay.
'is disturbed' indicates an anagram (the order of letters is disturbed).
'about' indicates putting letters inside.
'boris' with letters rearranged gives 'isobr'.
'isobr' enclosing 'a' is 'ISOBAR'.
(Other definitions for isobar that I've seen before include "(Map) line of constant atmospheric pressure" , "Line on a map connecting points of equal atmospheric pressure" , "Map-line showing places with equal air pressure" , "A Boris (anag.)" , "Met map pressure line" .)