Wife with no difficulty left unfaithful fellow? (6)
I believe the answer is:
weasel
'unfaithful fellow?' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'wife with no difficulty left' is the wordplay.
'wife' becomes 'w' (abbreviation).
'with no difficulty' becomes 'ease' (I've seen this in other clues).
'left' becomes 'l' (common abbreviation).
'w'+'ease'+'l'='WEASEL'
Can you help me to learn more?
(Other definitions for weasel that I've seen before include "Small fierce animal with slender body and reddish-brown fur" , "Double-dealer" , "Musteline animal" , "Small carnivore" , "Mammal related to stoat" .)