Boris a problem making line on map? (6)
I believe the answer is:
isobar
'line on map?' is the definition.
(line indicating pressure on a map)
'boris a problem' is the wordplay.
'problem' indicates anagramming the letters.
'boris'+'a'='borisa'
'borisa' anagrammed gives 'ISOBAR'.
'making' acts as a link.
(Other definitions for isobar that I've seen before include "Map line showing atmosphere pressure" , "Line on map showing barometric pressure" , "Line showing points having equal barometric pressure at a given time" , "A Boris (anag.)" , "meteorologist's line" .)