In the event, a back seat (6)
I believe the answer is:
saddle
'seat' is the definition.
(I know that saddle is a type of seat)
'in the event a back' is the wordplay.
'in the' means one lot of letters goes inside another.
'event' becomes 'sale' (promotional event).
'a back' becomes 'dd' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'sale' going around 'dd' is 'SADDLE'.
Can you help me to learn more?
(Other definitions for saddle that I've seen before include "Jockey's seat" , "Seat for a rider, horse or bicycle" , "Chafed by riding" , "Seat on horse, bicycle etc" , "Burden; seat" .)