That's that, and it's a Y! (3,2,5)
I believe the answer is:
end of story
'that's that' is the definition.
(I've seen this before)
'and it's a y' is the wordplay.
I cannot really see how this works, but
'and' could be 'n' (common abbreviation for 'and') and 'n' is found within the answer.
'y' is within the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is present 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 end of story that I've seen before include "When I close book" , "That's that" , "period" , "I won't reopen it" .)