When Rosie’s mad with us, it’s no joke! (7)
I believe the answer is:
serious
'no joke' is the definition.
(thesaurus)
'when rosie's mad with us' is the wordplay.
I cannot quite understand how this works, but
'when' could be 'e' (I've seen this in other clues) and 'e' is present in the answer.
'us' is found in the answer.
This may be the basis of clue (or it may be nonsense).
'it's' is the link.
Can you help me to learn more?
(Other definitions for serious that I've seen before include "Earnest, sincere" , "Solemn, thoughtful" , "Important - solemn" , "Earnest; vital" , "Earnest - important" .)