Rat seen in a river several years running (6)
I believe the answer is:
decade
'several years running' is the definition.
(a decade is ten years)
'rat seen in a river' is the wordplay.
'rat' becomes 'cad' (I've seen this in other clues).
'seen in' means one lot of letters goes inside another.
'a river' becomes 'dee' (River Dee).
'cad' put within 'dee' is 'DECADE'.
(Other definitions for decade that I've seen before include "The noughties maybe" , "The 1960s, for example" , "Period of ten years" , "Ten-year period" , "number of years" .)