Choose to enter river in buff (7)
I believe the answer is:
devotee
'buff' is the definition.
(I know that buff can be written as devotee)
'choose to enter river' is the wordplay.
'choose' becomes 'vote' (voting is a kind of choosing).
'to enter' is an insertion indicator.
'river' becomes 'Dee' (River Dee).
'vote' put inside 'dee' is 'DEVOTEE'.
'in' is the link.
(Other definitions for devotee that I've seen before include "Keen admirer" , "dedicated person" , "Fan, enthusiast" , "A very loyal or loving person" , "Zealous follower" .)