Plunged naked into river (4)
I believe the answer is:
ouse
'river' is the definition.
(Yorkshire river)
'plunged naked' is the wordplay.
'plunged' becomes 'soused' (sousing is a kind of plunging).
'naked' indicates the central letters (outside letters removed so centre is unclothed).
The central letters of 'soused' are 'OUSE'.
'into' acts as a link.
(Other definitions for ouse that I've seen before include "Lewes's river" , "bird has lost its tail" , "River at Bedford" , "York's river" , "Name of river in Sussex or Yorkshire" .)