Junior say sent up: we aren't upset about that (9)
I believe the answer is:
tweenager
'junior' is the definition.
(tweenager can mean a young teenager)
'say sent up we aren't upset about that' is the wordplay.
'say' becomes 'eg' (both can mean 'for example').
'sent up' is a reversal indicator.
'upset' indicates an anagram.
'about that' means one lot of letters goes inside another.
'eg' in reverse letter order is 'ge'.
'we'+'arent'='wearent'
'wearent' is an anagram of 'tweenar'.
'ge' going into 'tweenar' is 'TWEENAGER'.
(Other definitions for tweenager that I've seen before include "Precocious child" , "exasperating twelve-year-old" , "Youngster" , "(Informally) a child approaching the age of 13?" , "Possibly a ten-year-old" .)