Creature in middle of foaming river at York (5)
I believe the answer is:
mouse
'creature' is the definition.
(mouse is a kind of creature)
'middle of foaming river at york' is the wordplay.
'middle of' says to take the centre.
'river at york' becomes 'ouse' (I've seen this before).
The middle of 'foaming' is 'm'.
'm'+'ouse'='MOUSE'
'in' is the link.
(Other definitions for mouse that I've seen before include "Shy individual" , "Squeaky creature" , "Computer accessory" , "underhand office worker?" , "One sitting by computer" .)