Flash jewel ultimately claimed by identical leader (9)
I believe the answer is:
twinkling
'flash' is the definition.
(I know that flash can be written as twinkling)
'jewel ultimately claimed by identical leader' is the wordplay.
'ultimately' indicates one should take the final letters.
'claimed by' means one lot of letters goes inside another.
'identical' becomes 'twin' (I've seen this in another clue).
'leader' becomes 'king' (a king is a royal leader).
The final letter of 'jewel' is 'l'.
'twin'+'king'='twinking'
'l' placed inside 'twinking' is 'TWINKLING'.
(Other definitions for twinkling that I've seen before include "How stars appear" , "Bat of an eye" , "Sparkling like a star" , "Sparkling - moment" , "Shining intermittently" .)