To us, Rosie getting upset is no laughing matter (7)
I believe the answer is:
serious
'no laughing matter' is the definition.
(thesaurus)
'to us rosie getting upset' is the wordplay.
'getting upset' indicates an anagram.
'us'+'rosie'='usrosie'
'usrosie' with letters rearranged gives 'SERIOUS'.
'is' is the link.
(Other definitions for serious that I've seen before include "Earnest - important" , "Important - solemn" , "Stern" , "Important -- grave" , "Solemn, thoughtful" .)