Best part of planet I left, after launch? (5)
I believe the answer is:
elite
'best part' is the definition.
(I've seen this in another clue)
'of planet i left after launch?' is the wordplay.
'of' indicates a hidden word.
'launch?' shows that the letters should be reversed in order (I've seen 'launched' mean this (in a down clue, letters get launched up)).
'etile' is hidden in the letters of 'planet i left after'.
'etile' back-to-front is 'ELITE'.
(Other definitions for elite that I've seen before include "Top group f" , "A privileged or select group" , "Letter size; superior group" , "Pick of the bunch" , "Exclusive set" .)