Looking up in mirror at spook's heavenly body (4)
I believe the answer is:
star
'heavenly body' is the definition.
(I know that star is a type of heavenly body)
'looking up in mirror at spook's' is the wordplay.
'looking up' says the letters should be written in reverse (in a down clue, letters go up).
'in' indicates a hidden word.
'rats' is hidden in the letters of 'mirror at spooks'.
'rats' written backwards gives 'STAR'.
(Other definitions for star that I've seen before include "Major" , "Distant sun" , "Take the lead role" , "top player" , "may be a dwarf or a giant" .)