100 travelling around a line on map (7)
I believe the answer is:
contour
'a line on map' is the definition.
(I've seen this before)
'100 travelling' is the wordplay.
'100' becomes 'C' (roman numeral).
'travelling' becomes 'on tour' (on tour is a kind of travelling).
'c'+'ontour'='CONTOUR'
'around' is the link.
(Other definitions for contour that I've seen before include "Silhouette" , "Defining line" , "Map line of equal altitude" , "Outline, profile" , "Line on map indicating height" .)