Upset Teresa enunciates showing anxiety (6)
I believe the answer is:
unease
'anxiety' is the definition.
(anxious or nervous condition)
'upset teresa enunciates showing' is the wordplay.
'upset' says the letters should be written backwards.
'showing' says the answer is hidden in the clue (the answer is shown in the clue).
'esaenu' is hidden within 'teresa enunciates'.
'esaenu' back-to-front is 'UNEASE'.
(Other definitions for unease that I've seen before include "Discomfort or apprehension" , "State of anxiety" , "Anxiety, discontent" , "State of worry and lack of confidence" , "Disquiet" .)