Things go wrong, but luggage turns up (5,7)
I believe the answer is:
stuff happens
'luggage turns up' is the definition.
I don't know anything about this answer so I can't judge whether this works.
'things go wrong' is the wordplay.
'things' becomes 'stuff' (I've seen this in another clue).
'go wrong' becomes 'happens' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'stuff'+'happens'='STUFF HAPPENS'
'but' acts as a link.
Can you help me to learn more?
(Another definition for stuff happens that I've seen is " Rumsfeld's comment on collateral damage".)