Animal losing tail in river (3)
I believe the answer is:
dee
'river' is the definition.
(River Dee)
'animal losing tail' is the wordplay.
'animal' becomes 'deer' (deer is a type of animal).
'losing tail' means to remove the last letter.
'deer' with its final letter taken away is 'DEE'.
'in' acts as a link.
(Other definitions for dee that I've seen before include "Aberdeenshire river" , "Name of many rivers" , "Chester's river" , "Balmoral's river" , "Scottish river" .)