Sparkling double sovereign covering pound (9)
I believe the answer is:
twinkling
'sparkling' is the definition.
(I've seen this before)
'double sovereign covering pound' is the wordplay.
'double' becomes 'twin' (twin can mean someone's double).
'sovereign' becomes 'king' (king is a kind of sovereign).
'covering' indicates putting letters inside.
'pound' becomes 'L' (abbreviation e.g. Lsd - pounds, shillings, pence).
'king' placed around 'l' is 'kling'.
'twin'+'kling'='TWINKLING'
(Other definitions for twinkling that I've seen before include "Jiffy" , "Bat of an eye" , "Instant" , "How stars appear" , "for a short time" .)