Go back in coach, dummy! (5)
I believe the answer is:
bogus
'dummy' is the definition.
(I've seen this before)
'go back in coach' is the wordplay.
'back' says the letters should be written in reverse.
'in' indicates putting letters inside.
'coach' becomes 'bus' (synonyms).
'go' backwards is 'og'.
'og' placed into 'bus' is 'BOGUS'.
(Other definitions for bogus that I've seen before include "Dummy" , "Scribbled drawing" , "Fraudulent, fake" , "Sham, counterfeit" , "that's not real" .)