Declare Tessa’s upset about Romeo (6)
I believe the answer is:
assert
'declare' is the definition.
(asserting is a kind of declaring)
'tessa's upset about romeo' is the wordplay.
'upset' says the letters should be written in reverse.
'about' indicates putting letters inside.
'romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'tessa' reversed gives 'asset'.
'asset' placed around 'r' is 'ASSERT'.
(Other definitions for assert that I've seen before include "protest" , "Declare, claim" , "Forcefully state" , "State firmly and with conviction" , "Confidently say" .)