British stream ain't unfortunately sparkling (9)
I believe the answer is:
brilliant
'sparkling' is the definition.
(brilliant can mean sparkling or glistening)
'british stream ain't unfortunately' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'stream' becomes 'rill' (rill is a kind of stream).
'unfortunately' indicates anagramming the letters (a bad or unfortunate spelling).
'aint' with letters rearranged gives 'iant'.
'b'+'rill'+'iant'='BRILLIANT'
(Other definitions for brilliant that I've seen before include "Lustrous" , "'Wonderful, dazzling (9)'" , "Glittering" , "Very bright; marvellous" , "Small type" .)