Whopper served up in Marseilles (3)
I believe the answer is:
lie
'whopper' is the definition.
(I've seen this before)
'up in marseilles' is the wordplay.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'in' indicates a hidden word.
'eil' is hidden in the letters of 'marseilles'.
'eil' written backwards gives 'LIE'.
'served' is the link.
(Other definitions for lie that I've seen before include "create fiction" , "Perjure" , "False statement" , "found" , "Assume a horizontal position" .)