Nasty, as a sun and planet explode (10)
I believe the answer is:
unpleasant
'nasty' is the definition.
('unpleasant' can be a synonym of 'nasty')
'a sun and planet explode' is the wordplay.
'and' says to put letters next to each other (I've seen this in other clues).
'explode' is an anagram indicator.
'a'+'sun'='asun'
'asun' after 'planet' is 'planetasun'.
'planetasun' is an anagram of 'UNPLEASANT'.
'as' acts as a link.
(Other definitions for unpleasant that I've seen before include "Not nice" , "Obnoxious" , "Nasty" , "like Mrs Warren's Profession?" , "rude" .)