Almost tumble in Cornish river (3)
I believe the answer is:
fal
'cornish river' is the definition.
(River Fal)
'almost tumble' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'tumble' becomes 'fall' (synonyms).
'fall' with its last letter taken off is 'FAL'.
'in' acts as a link.
(Other definitions for fal that I've seen before include "Cornish river" , "Flower" .)