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