Cost of excursion, including travel (8)
I believe the answer is:
outgoing
'cost' is the definition.
'excursion including travel' is the wordplay.
'excursion' becomes 'outing' ('outing' can be a synonym of 'excursion').
'including' means one lot of letters goes inside another.
'travel' becomes 'go' ('go' can be a synonym of 'travel'**).
'outing' going around 'go' is 'OUTGOING'.
'of' acts as a link.
(Other definitions for outgoing that I've seen before include "At ease in talking to others, extroverted" , "Extrovert" , "Departing" , "open" , "Friendly - retiring" .)