A biro's possibly used to make line on map (6)
I believe the answer is:
isobar
'line on map' is the definition.
(I've seen this before)
'a biro's possibly' is the wordplay.
'possibly' indicates anagramming the letters.
'a'+'biros'='abiros'
'abiros' with letters rearranged gives 'ISOBAR'.
'used to make' is the link.
(Other definitions for isobar that I've seen before include "Weather map feature" , "It indicates pressure" , "feature on some maps?" , "As brio (anag)" , "It connects points of equal air pressure" .)