Brilliant around stream (4)
I believe the answer is:
rill
'stream' is the definition.
(I know that rill is a type of stream)
'brilliant around' is the wordplay.
'around' indicates the answer is hidden within the clue.
'RILL' can be found hidden inside 'brilliant'.
(Other definitions for rill that I've seen before include "Small brook" , "Flowing water" , "Little stream" , "Flow of water" , "Brooklet" .)