If the other crossword isn't finished it's not difficult to see why (6)
I believe the answer is:
simple
'why' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for simple that I've seen before include "A piece of cake" , "Effortless" , "The sort of interest" , "Plant with medicinal properties" , "Uncomplicated; herb" .)