Ticket seller has teacher back as a traveller (7)
I believe the answer is:
tourist
'a traveller' is the definition.
(I know that tourist is a type of traveller)
'ticket seller has teacher back' is the wordplay.
'ticket seller' becomes 'tout' (as in a ticket tout).
'has' is an insertion indicator.
'teacher' becomes 'sir' (a male teacher might be addressed as 'sir').
'back' is a reversal indicator.
'sir' reversed gives 'ris'.
'tout' placed around 'ris' is 'TOURIST'.
'as' is the link.
(Other definitions for tourist that I've seen before include "someone on holiday" , "Cheap travel class" , "Visiting sightseer" , "Someone who travels for pleasure" , "Sightseeing holidaymaker" .)