Greek character departs late, unfortunately (5)
I believe the answer is:
delta
'greek character' is the definition.
(letter from the Greek alphabet)
'departs late unfortunately' is the wordplay.
'departs' becomes 'd' (common abbreviation e.g. train timetables).
'unfortunately' indicates an anagram (a bad or unfortunate spelling).
'late' with letters rearranged gives 'elta'.
'd'+'elta'='DELTA'
(Other definitions for delta that I've seen before include "The Greek D (5)" , "Branching river mouth" , "Greek letter in the shape of a triangle" , "Area where a river divides before entering the sea" , "Triangular patch of land at mouth of river" .)