Go back in coach that's fake (5)
I believe the answer is:
bogus
'fake' is the definition.
('bogus' can be a synonym of 'fake')
'go back in coach' is the wordplay.
'back' says the letters should be written backwards.
'in' means one lot of letters goes inside another.
'coach' becomes 'bus' ('bus' can be a synonym of 'coach'**).
'go' back-to-front is 'og'.
'og' put inside 'bus' is 'BOGUS'.
'that's' is the link.
(Other definitions for bogus that I've seen before include "Sham, false" , "Pretended" , "taking one for a ride?" , "Fictitious" , "Phoney, not genuine" .)