One packed in best bag for flight (9)
I believe the answer is:
staircase
'flight' is the definition.
(a flight of stairs)
'one packed in best bag' is the wordplay.
'one' becomes 'i' (roman numeral).
'packed in' means one lot of letters goes inside another.
'best' becomes 'star' (star can mean the best or most excellent).
'bag' becomes 'case' (I have seen 'Overnight bag' mean 'case' so perhaps 'bag' could also mean 'case').
'i' inserted into 'star' is 'stair'.
'stair'+'case'='STAIRCASE'
'for' is the link.
(Other definitions for staircase that I've seen before include "It's Caesar (anag.)" , "Set of steps" , "Route between floors" , "exited via this?" , "Situation of flight" .)