Boar is travelling line on a map (6)
I believe the answer is:
isobar
'line on a map' is the definition.
(line indicating pressure on a map)
'boar is travelling' is the wordplay.
'travelling' indicates anagramming the letters.
'boar'+'is'='boaris'
'boaris' is an anagram of 'ISOBAR'.
(Other definitions for isobar that I've seen before include "As brio (anag)" , "feature on some maps?" , "Line in front" , "Map line showing atmosphere pressure" , "Line on map showing barometric pressure" .)