Greek character in dark apparel (5)
I believe the answer is:
kappa
'greek character' is the definition.
(letter from the Greek alphabet)
'in dark apparel' is the wordplay.
'in' says the answer is hidden in the clue.
'KAPPA' is hidden in the letters of 'dark apparel'.
(Other definitions for kappa that I've seen before include "Greek K" , "10th Greek letter" , "letter from abroad" , "Letter before lambda" , "Tenth Greek letter" .)