Greek character dealt deviously (5)
I believe the answer is:
delta
'greek character' is the definition.
(letter from the Greek alphabet)
'dealt deviously' is the wordplay.
'deviously' is an anagram indicator (I've seen this in other clues).
'dealt' anagrammed gives 'DELTA'.
(Other definitions for delta that I've seen before include "Letter - where river divides" , "Split river mouth system" , "Extended area at river mouth" , "The Greek D (5)" , "Fourth letter of the ancient Greek alphabet: river feature" .)