Body that’s downgraded parking on most of airport (5)
I believe the answer is:
pluto
'body' is the definition.
(I have seen 'heavenly body' mean 'pluto' so perhaps 'body' could also mean 'pluto')
'parking on most of airport' is the wordplay.
'parking' becomes 'p' (abbreviation on road signs).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'most of' means to remove the last letter.
'airport' becomes 'Luton' (London Luton Airport).
'luton' with its last letter taken away is 'luto'.
'p'+'luto'='PLUTO'
'that's downgraded' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for pluto that I've seen before include "underworld boss" , "Solar System's smallest planet" , "heavenly body" , "Sun's remote orbiter" , "Disney character - Underworld god - planet" .)