Under no circumstances is Dad leaving cheese (4)
I believe the answer is:
neer
'dad leaving cheese' is the definition.
I can't tell whether this definition defines the answer.
'under no circumstances' is the wordplay.
'under' says to put letters next to each other (in a down clue, some letters go below others).
'no' becomes 'n' (common abbreviation).
'circumstances' becomes 'eer' (I can't explain this - if you can you should give a lot more credence to this answer).
'n'+'eer' is 'NEER'.
'is' is the link.
Can you help me to learn more?
(Other definitions for neer that I've seen before include "On no occasion" , "Of course not" , "At no time (poetic)" , "At no time (literary)" .)