Advertises Spielberg classic, wearing best suit (8)
I believe the answer is:
trumpets
'advertises' is the definition.
(trumpet can mean to advertise or publicise)
'spielberg classic wearing best suit' is the wordplay.
'spielberg classic' becomes 'et' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'wearing' means one lot of letters goes inside another.
'best suit' becomes 'trumps' (card-playing term).
'et' put inside 'trumps' is 'TRUMPETS'.
Can you help me to learn more?
(Other definitions for trumpets that I've seen before include "Brass musical instruments" , "Brass wind instruments" , "Proclaims loudly like elephant sounds" .)