Rotter in river for long time (6)
I believe the answer is:
decade
'long time' is the definition.
The definition and answer can be both related to time as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'rotter in river' is the wordplay.
'rotter' becomes 'cad' (I've seen this in another clue).
'in' means one lot of letters goes inside another.
'river' becomes 'dee' (River Dee).
'cad' inserted into '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 1960s, for example" , "Ten-year period" , "Ten consecutive years" , "Ten-year span" , "The noughties maybe" .)