Outline, for the many on the trip (7)
I believe the answer is:
contour
'outline' is the definition.
(contouring is a kind of outlining)
'many on the trip' is the wordplay.
'many' becomes 'c' (C is the Roman numeral for 100).
'the trip' becomes 'tour' (a journey or expedition).
'c'+'on'+'tour'='CONTOUR'
'for the' is the link.
(Other definitions for contour that I've seen before include "Line on map indicating height" , "Map line of equal altitude" , "Silhouette" , "Outline (of land)" , "Outline, shape" .)