Bagel not having "gross" filling (6)

I believe the answer is:
nought
'bagel' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'not having gross filling' is the wordplay.
'having gross' becomes 'ugh' (I am not sure about this - if you are sure you should believe this answer much more).
'filling' means one lot of letters goes inside another.
'not' going around 'ugh' is 'NOUGHT'.
Can you help me to learn more?
(Other definitions for nought that I've seen before include "Zilch" , "The figure 0" , "duck" , "Love" , "The digit 0" .)
