Greek character held by some gangsters (5)
I believe the answer is:
omega
'greek character' is the definition.
(letter from the Greek alphabet)
'held by some gangsters' is the wordplay.
'held by' says the answer is hidden in the clue.
'OMEGA' is hidden in the letters of 'some gangsters'.
(Other definitions for omega that I've seen before include "24th Greek letter" , "Ultimate letter" , "Last letter of the Greek alphabet" , "Conclusion" , "long letter" .)