Rosie's upset over us being earnest (7)
I believe the answer is:
serious
'earnest' is the definition.
(similar in meaning)
'rosie's upset over us' is the wordplay.
'upset' indicates an anagram.
'over' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'rosie' anagrammed gives 'serio'.
'serio'+'us'='SERIOUS'
'being' acts as a link.
(Other definitions for serious that I've seen before include "Not light or frivolous" , "Important - solemn" , "Earnest - important" , "Stern" , "Important -- grave" .)