Go back on coach, fake! (5)
I believe the answer is:
bogus
'fake' is the definition.
(I know that fake can be written as bogus)
'go back on coach' is the wordplay.
'back' says the letters should be written in reverse.
'on' means one lot of letters goes inside another.
'coach' becomes 'bus' (synonyms).
'go' in reverse letter order is 'og'.
'og' inserted into 'bus' is 'BOGUS'.
(Other definitions for bogus that I've seen before include "Fake, phoney" , "Fictitious" , "Mock" , "Scribbled drawing" , "Fake, sham" .)