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