Carnival route initially appears in complicated diagrams (5,4)
I believe the answer is:
mardi gras
'carnival' is the definition.
(Mardi Gras is an example)
'route initially appears in complicated diagrams' is the wordplay.
'initially' indicates taking the first letters.
'appears in' is an insertion indicator.
'complicated' indicates anagramming the letters.
The initial letter of 'route' is 'r'.
'diagrams' with letters rearranged gives 'mardigas'.
'r' inserted inside 'mardigas' is 'MARDI GRAS'.
(Other definitions for mardi gras that I've seen before include "festive occasion" , "Shrove Tuesday celebration" , "Shrove Tuesday carnival" , "Shrove Tuesday festival" , "Time for carnival" .)