Rat in river for some years (6)
I believe the answer is:
decade
'some years' is the definition.
(I've seen this before)
'rat in river' is the wordplay.
'rat' becomes 'cad' (I've seen this in other clues).
'in' indicates putting letters inside.
'river' becomes 'dee' (River Dee).
'cad' going into 'dee' is 'DECADE'.
'for' is the link.
(Other definitions for decade that I've seen before include "Period of ten years" , "Ten-year span" , "Time period" , "Series of 10 years" , "number of years" .)