Worried about Greek character (3)
I believe the answer is:
eta
'greek character' is the definition.
(letter from the Greek alphabet)
'worried about' is the wordplay.
'worried' becomes 'ate' (something worrying may eat someone up).
'about' shows that the letters should be reversed in order.
'ate' in reverse letter order is 'ETA'.
(Other definitions for eta that I've seen before include "letter from Greece?" , "Foreign letter" , "Letter of Greek alphabet" , "Seventh Greek letter" , "When's end of journey predicted" .)