Making pencil unusable is the last straw (8,5)
I believe the answer is:
breaking point
'straw' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
I don't understand how the remainder of the clue works.
(Other definitions for breaking point that I've seen before include "Moment at which something gives way" , "The end of one's tether?" , "Critical moment for collapse" .)