Kind of sweet to ring enemy up? Don't be daft! (4,3,2)
I believe the answer is:
come off it
'don't be daft' is the definition.
I can't judge whether this defines the answer.
'kind of sweet to ring enemy up?' is the wordplay.
'kind of sweet' becomes 'comfit' ().
'to ring' is an insertion indicator.
'enemy' becomes 'foe' (synonyms).
'up?' says the letters should be written backwards (in down clue: letters go upwards).
'foe' back-to-front is 'eof'.
'comfit' enclosing 'eof' is 'COME OFF IT!'.
Can you help me to learn more?
(Other definitions for come off it that I've seen before include "Don't be ridiculous!" , "stop fooling" , "You're mad!" , "Don't talk rubbish" , "Don't make me laugh" .)