As a rotter, one is not in good odour (5)
I believe the answer is:
skunk
'good odour' is the definition.
I know that 'odour' relates to this answer. I also know that both the answer and definition are singular nouns
'as a rotter one is not' is the wordplay.
I cannot really see how this works, but
'one' could be 'un' (northern English dialect form of 'one') and 'un' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'in' acts as a link.
Can you help me to learn more?
(Other definitions for skunk that I've seen before include "Marijuana - scumbag" , "American animal notorious for its bad smell" , "Strong pot" , "rat" , "Evil-smelling animal of N America" .)