Crossword Genius

A biro's deployed to make a line on a weather map (6)

Ross

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" .)

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