Tragic end to a morning on the river (3)
I believe the answer is:
cam
'the river' is the definition.
(river in Cambridge, England)
'tragic end to a morning on' is the wordplay.
'end' indicates one should take the final letters.
'to a morning' becomes 'am'.
'on' means one lot of letters go next to another.
The final letter of 'tragic' is 'c'.
'c'+'am'='CAM'
(Other definitions for cam that I've seen before include "East Anglian river" , "Rotating mechanism" , "running into Great Ouse?" , "Projection on rotating shaft imparting motion" , "River also called Granta" .)