River cascade running short (3)
I believe the answer is:
fal
'river' is the definition.
(Cornish river)
'cascade running short' is the wordplay.
'cascade' becomes 'fall' (I have seen 'Cascade of water ' mean 'fall' so perhaps 'cascade' could also mean 'fall').
'running short' means to remove the last letter.
'fall' with its last letter removed is 'FAL'.
(Other definitions for fal that I've seen before include "Cornish river" , "Flower" .)