A cut — concerning in my union (9)
I believe the answer is:
matrimony
'union' is the definition.
(I know that union can be written as matrimony)
'a cut concerning in my' is the wordplay.
'a cut' becomes 'matri' (I can't explain this - if you can you should give a lot more credence to this answer).
'concerning' becomes 'on' (both can mean 'in relation to').
'in' means one lot of letters goes inside another.
'on' inserted within 'my' is 'mony'.
'matri'+'mony'='MATRIMONY'
(Other definitions for matrimony that I've seen before include "State of being married" , "wedding ceremony" , "The state of wedlock" , "Nuptials" .)