Unauthorised passenger was to, after changing course (8)
I believe the answer is:
stowaway
'unauthorised passenger' is the definition.
(eg on a ship)
'was to after changing course' is the wordplay.
'after' says to put letters next to each other.
'changing' is an anagram indicator.
'course' becomes 'way' (way is a kind of course).
'was'+'to'='wasto'
'wasto' with letters rearranged gives 'stowa'.
'stowa'+'way'='STOWAWAY'
(Other definitions for stowaway that I've seen before include "Concealed traveller" , "pack" , "Travel secretly" , "One who takes a sea trip without paying" , "Fare-dodger" .)