New diagrams to cover start of route for carnival (5,4)
I believe the answer is:
mardi gras
'carnival' is the definition.
(I know that Mardi Gras is a type of carnival)
'new diagrams to cover start of route' is the wordplay.
'new' is an anagram indicator.
'to cover' means one lot of letters goes inside another.
'start of' says to take the initial letters.
The first letter of 'route' is 'r'.
'diagrams' anagrammed gives 'madigras'.
'madigras' going around 'r' is 'MARDI GRAS'.
'for' is the link.
(Other definitions for mardi gras that I've seen before include "festive occasion" , "Shrove Tuesday carnival" , "The lead up to fast" , "pre-Lent festival" , "Carnival (day)" .)