Gangster (7)
(Other definitions for mobster that I've seen before include "Organised criminal" , "Hoodlum" , "Mafioso" , "Criminal gang member" , "Hood" .)
'gangster' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for hoodlum that I've seen before include "Tearaway" , "Yobbo" , "Young thug or criminal" , "tough!" , "Small-time criminal" .)