Line on map confuses Boris first of all (6)
I believe the answer is:
isobar
'line on map' is the definition.
(line indicating pressure on a map)
'confuses boris first of all' is the wordplay.
'confuses' indicates anagramming the letters.
'first of all' becomes 'a' (1st letter of 'all').
'boris'+'a'='borisa'
'borisa' with letters rearranged gives 'ISOBAR'.
(Other definitions for isobar that I've seen before include "Met map pressure line" , "(Map) line of constant atmospheric pressure" , "Line on map connecting areas of equal pressure" , "Map line showing atmosphere pressure" , "Line of equal barometric pressure on a map" .)