A biro's deployed to make 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 in another clue)
'a biro's deployed' is the wordplay.
'deployed' indicates an anagram.
'a'+'biros'='abiros'
'abiros' is an anagram of 'ISOBAR'.
'to make' is the link.
(Other definitions for isobar that I've seen before include "Line on a map connecting points of equal atmospheric pressure" , "Line on map connecting areas of equal pressure" , "feature on some maps?" , "depression indicator?" , "Line of equal barometric pressure on a map" .)