Box used by king and queen containing large diamond? (8)
I believe the answer is:
sparkler
'diamond?' is the definition.
(something which sparkles)
'box used by king and queen containing large' is the wordplay.
'box' becomes 'spar' (sparring is a kind of boxing).
'used by' says to put letters next to each other.
'king and queen' becomes 'ker' (K for King and ER for Elizabeth Regina).
'containing' is an insertion indicator.
'large' becomes 'L' (eg in clothes sizes).
'ker' enclosing 'l' is 'kler'.
'spar'+'kler'='SPARKLER'
(Other definitions for sparkler that I've seen before include "Firework - precious stone" , "Possibly brilliant" , "Hand-held firework" , "Firework or diamond ring" , "Bright gem" .)