A throne to be destroyed? Not this one! (7)
I believe the answer is:
another
'not this one' is the definition.
(not this one, ie another one)
'a throne to be destroyed?' is the wordplay.
'to be destroyed?' indicates anagramming the letters.
'throne' anagrammed gives 'nother'.
'a'+'nother'='ANOTHER'
(Other definitions for another that I've seen before include "second" , "A different one" , "Player or person not yet named or selected" , "No heart (anag.)" , "encore?" .)