Start doing the Mexican wave as Mardi Gras ends (7)
I believe the answer is:
genesis
'start' is the definition.
(thesaurus)
'doing the mexican wave as mardi gras ends' is the wordplay.
'ends' indicates one should take the final letters.
The final letters of 'doing the mexican wave as mardi gras' are 'GENESIS'.
(Other definitions for genesis that I've seen before include "Origin; Bible book" , "First book of the Old Testament" , "the start of something" , "name of group" , "First book of the Bible" .)