When Rosie's upset over us, it's no laughing matter (7)
I believe the answer is:
serious
'no laughing matter' is the definition.
(thesaurus)
'when rosie's upset over' is the wordplay.
'when' becomes 'us' (I can't explain this - if you can you should give a lot more credence to this answer).
'upset' indicates anagramming the letters.
'over' says to put letters next to each other (in a down clue, some letters go over others).
'rosie' anagrammed gives 'serio'.
'us' put after 'serio' is 'SERIOUS'.
'us it's' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for serious that I've seen before include "Stern" , "Earnest - important" , "Important -- grave" , "Solemn, thoughtful" , "Important - solemn" .)