Maybe sixties rat in river (6)
I believe the answer is:
decade
'maybe sixties' is the definition.
(I know that 1960s is a decade)
'rat in river' is the wordplay.
'rat' becomes 'cad' (I've seen this in other clues).
'in' is an insertion indicator.
'river' becomes 'Dee' (River Dee).
'cad' inserted inside 'dee' is 'DECADE'.
(Other definitions for decade that I've seen before include "Ten consecutive years" , "The 1960s, for example" , "Time period" , "Period of ten years" , "The noughties, say" .)