In short, you and I are past (4)
I believe the answer is:
were
'are past' is the definition.
The answer and definition can be both to do with ownership as well as being past participle verbs.
Perhaps there's a link between them I don't understand?
'in short you and i' is the wordplay.
'in' means one lot of letters goes inside another.
'short' becomes 'wee' (I've seen this in other clues).
'you and i' becomes 'r' (I can't justify this - if you can you should believe this answer much more).
'wee' placed around 'r' is 'WERE'.
Can you help me to learn more?
(Other definitions for were that I've seen before include "Existed in the past" , "'Are, in the past (4)'" , "SECOND PART OF 9" , "Used to be" , "(You/we/they) existed" .)