Exciting thing about a stream (9)
I believe the answer is:
thrilling
'exciting' is the definition.
(thrilling is a kind of exciting)
'thing about a stream' is the wordplay.
'about' is an insertion indicator.
'a stream' becomes 'rill' (rill is a kind of stream**).
'thing' going around 'rill' is 'THRILLING'.
(Other definitions for thrilling that I've seen before include "Engrossing" , "Hair-raising" , "Sensational" , "Extremely exciting" , "Causing pleasurable excitement" .)