Jewel box presented by King Lear, Cordelia ultimately rejected (8)
I believe the answer is:
sparkler
'jewel' is the definition.
(I know that sparkler is a type of diamond)
'box presented by king lear cordelia ultimately rejected' is the wordplay.
I cannot really see how this works, but
'box' could be 'ark' (ark is a kind of box) and 'ark' is located in the answer.
'king' could be 'er' (abbreviation for Edward Rex, various English kings) and 'er' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for sparkler that I've seen before include "Hand-held firework" , "Eg, a diamond" , "Firework - precious stone" , "Jewel" , "Bright gem" .)