Underneath it all, one's reason is disintegrating, having reached the end of one's tether, perhaps (8,5)
I believe the answer is:
breaking point
'tether perhaps' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'underneath it all one's reason is disintegrating having reached the end of one's' is the wordplay.
I cannot quite understand how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found in the answer.
'end' could be 'break' (breaking is a kind of ending) and 'break' is found in the answer.
'of' could be 'o' and 'o' is present in the answer.
'one' could be 'i' (Roman numeral) and 'i' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for breaking point that I've seen before include "The end of one's tether?" , "Critical moment for collapse" , "Moment at which something gives way" .)