As such, there's no difference between best and worst (4)
I believe the answer is:
verb
'as' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for verb that I've seen before include "'In grammar, it denotes action (4)'" , "Word such as run, make, or do" , "Word that denotes an action or a state" , "The predicate of a sentence" , "Type of word indicating action" .)