Heavenly body starts off pretty luscious, until tragically old (5)
I believe the answer is:
pluto
'heavenly body' is the definition.
(Pluto is an example)
'starts off pretty luscious until tragically old' is the wordplay.
'starts off' indicates taking the first letters.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
The first letters of 'pretty luscious until tragically' is 'plut'.
'plut'+'o'='PLUTO'
(Other definitions for pluto that I've seen before include "Hollywood creation" , "that is submerged under water" , "Ninth planet of solar system" , "Most remote planet" , "Infernal ruler" .)