Exciting fact about flowing water (9)
I believe the answer is:
thrilling
'exciting' is the definition.
(thrilling is a kind of exciting)
'fact about flowing water' is the wordplay.
'fact' becomes 'thing'.
'about' indicates putting letters inside.
'flowing water' becomes 'rill' (I've seen this before).
'thing' placed around 'rill' is 'THRILLING'.
(Other definitions for thrilling that I've seen before include "Hair-raising" , "Extremely exciting" , "Gripping" , "Causing pleasurable excitement" , "Engrossing" .)