What you might say brings a smile to all sides today (6)
I believe the answer is:
cheese
'today' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for cheese that I've seen before include "Food made from milk curds" , "Feta, for example" , "Product of milk curds" , "Eg, cheddar" , "product of cottage industry?" .)