Silly to upset Annie (5)
I believe the answer is:
inane
'silly' is the definition.
(I've seen this before)
'upset annie' is the wordplay.
'upset' is an anagram indicator.
'annie' is an anagram of 'INANE'.
'to' is the link.
(Other definitions for inane that I've seen before include "Unintelligent" , "unimaginative" , "Mindlessly silly" , "Without meaning or point" , "Fatuous, mindless" .)