I must pack principal luggage for flight (9)
I believe the answer is:
staircase
'flight' is the definition.
(a flight of stairs)
'i must pack principal luggage' is the wordplay.
'must pack' is an insertion indicator (inserted letters get packed or contained).
'principal' becomes 'star' (synonyms).
'luggage' becomes 'case' (I've seen this in other clues).
'i' going inside 'star' is 'stair'.
'stair'+'case'='STAIRCASE'
'for' acts as a link.
(Other definitions for staircase that I've seen before include "It's Caesar (anag.)" , "Way between floors" , "exited via this?" , "Route between floors" , "Means of moving from one floor to another" .)