River of Ukraine seems endless, initially (4)
I believe the answer is:
ouse
'river' is the definition.
(Yorkshire river)
'of ukraine seems endless initially' is the wordplay.
'initially' indicates taking the first letters.
The initial letters of 'of ukraine seems endless' is 'OUSE'.
(Other definitions for ouse that I've seen before include "Runner" , "River at Bedford" , "Name of river in Sussex or Yorkshire" , "River at Lewes (Sussex)" , "Lewes's river" .)