Beat bad, corrupt Greek character (6)
I believe the answer is:
lambda
'greek character' is the definition.
(letter from the Greek alphabet)
'beat bad corrupt' is the wordplay.
'beat' becomes 'lam' (I've seen this before).
'corrupt' is an anagram indicator.
'bad' with letters rearranged gives 'bda'.
'lam'+'bda'='LAMBDA'
(Other definitions for lambda that I've seen before include "Greek character" , "Greek letter - am bald (anag)" , "Steps taken" , "Eleventh Greek letter" , "Greek letter L" .)