A biro's making a line on a weather map (6)
I believe the answer is:
isobar
'a line on a weather map' is the definition.
(I've seen this before)
'a biro's making' is the wordplay.
'making' indicates anagramming the letters.
'a'+'biros'='abiros'
'abiros' anagrammed gives 'ISOBAR'.
(Other definitions for isobar that I've seen before include "Weather-map line" , "Line of equal barometric pressure on a map" , "Weather map feature" , "Contour" , "meteorologist's line" .)