Rotter in river for extended period (6)
I believe the answer is:
decade
'extended period' is the definition.
'decade' can be an answer for 'period' (decade is a kind of period). I'm not certain of the 'extended' bit.
'rotter in river' is the wordplay.
'rotter' becomes 'cad' (I've seen this in another clue).
'in' indicates putting letters inside.
'river' becomes 'dee' (River Dee).
'cad' put within 'dee' is 'DECADE'.
'for' acts as a link.
Can you help me to learn more?
(Other definitions for decade that I've seen before include "The noughties maybe" , "Ten-year span" , "The noughties, say" , "Series of 10 years" , "number of years" .)