What a Greek can write was worrying on reflection (3)
I believe the answer is:
eta
'what a greek can write' is the definition.
(eta is a Greek letter)
'was worrying on reflection' is the wordplay.
'was worrying' becomes 'ate' (eat can mean to worry or distress).
'on reflection' says the letters should be written in reverse.
'ate' backwards is 'ETA'.
(Other definitions for eta that I've seen before include "Greek letter E; estimated time of arrival (abbr.)" , "Letter of Greek alphabet" , "Member of Greek set a" , "Something like 10 [RHO]" , "Arrival roughly indicated" .)