Key tale about Greek character (5)
I believe the answer is:
delta
'greek character' is the definition.
(letter from the Greek alphabet)
'key tale about' is the wordplay.
'key' becomes 'd' (I've seen this in other clues**).
'about' is an anagram indicator.
'tale' with letters rearranged gives 'elta'.
'd'+'elta'='DELTA'
(Other definitions for delta that I've seen before include "Estuary area where river has split" , "Daedalus started with this" , "one's often seen at mouth of river" , "Greek letter in the shape of a triangle" , "Type of wing" .)