Greek character held by loathsome gang (5)
I believe the answer is:
omega
'greek character' is the definition.
(letter from the Greek alphabet)
'held by loathsome gang' is the wordplay.
'held by' indicates a hidden word.
'OMEGA' is hidden in the letters of 'loathsome gang'.
(Other definitions for omega that I've seen before include "long letter" , "Foreign character" , "Greek long 'o'" , "Traditional story" , "Conclusion" .)