Splendid salesman aboard bus heading west (6)
I believe the answer is:
superb
'splendid' is the definition.
(both can mean excellent)
'salesman aboard bus heading west' is the wordplay.
'salesman' becomes 'rep' (sales representative).
'aboard' means one lot of letters goes inside another.
'heading west' says the letters should be written backwards.
'rep' put into 'bus' is 'brepus'.
'brepus' backwards is 'SUPERB'.
(Other definitions for superb that I've seen before include "Of surpassing excellence" , "Surpassingly good" , "Tiptop" , "Really excellent, brilliant" , "'Excellent, splendid (6)'" .)