Great coach carrying agent up (6)
I believe the answer is:
superb
'great' is the definition.
(thesaurus)
'coach carrying agent up' is the wordplay.
'coach' becomes 'bus' ('bus' can be a synonym of 'coach'**).
'carrying' means one lot of letters goes inside another.
'agent' becomes 'rep' (short for representative).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'bus' going around 'rep' is 'brepus'.
'brepus' backwards is 'SUPERB'.
(Other definitions for superb that I've seen before include "choice" , "Really excellent, brilliant" , "Of surpassing excellence" , "Wonderful, really great" , "Excellent, magnificent" .)