Group is mostly keen about new holiday itinerary (9)
I believe the answer is:
entourage
'group' is the definition.
(I know that entourage is a type of assemblage)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for entourage that I've seen before include "Support team" , "Staff" , "Ego nature found in accompanying group" , "Retinue, cortege" , "A star may have one" .)