River put to no practical purpose? (4)
I believe the answer is:
ouse
'river' is the definition.
(Yorkshire river)
'to no practical purpose?' is the wordplay.
'to no' becomes 'O' (resembles 0 - 'no' can mean 'zero').
'practical purpose?' becomes 'use' (I've seen this in another clue).
'o'+'use'='OUSE'
'put' acts as a link.
(Other definitions for ouse that I've seen before include "River that joins the Trent to form the Humber" , "River of 1 across" , "Lewes's river" , "One of several rivers" , "Name of several English rivers" .)