Characters in Cosa Nostra developed a line of work (5)
I believe the answer is:
trade
'a line of work' is the definition.
(trade is a kind of line of work)
'characters in cosa nostra developed' is the wordplay.
'characters in' indicates the answer is hidden within the clue.
'TRADE' is hidden in the letters of 'cosa nostra developed'.
(Other definitions for trade that I've seen before include "A skilled craft to earn living" , "Barter, do business" , "Commercial dealings" , "Buying and selling of goods" , "Customers" .)