There's not much on Sunday or Monday (4)

I believe the answer is:

whit

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'monday' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for whit that I've seen before include "Very small part - Christian festival" , "Iota" , "Tiny amount; a church Sunday" , "Pentecost (abbrev.)" , "Festival week" .)

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