Gangster harbours anxiety (5)
I believe the answer is:
angst
'anxiety' is the definition.
(Angst is an example)
'gangster harbours' is the wordplay.
'harbours' indicates the answer is hidden within the clue.
'ANGST' is hidden in the letters of 'gangster'.
(Other definitions for angst that I've seen before include "Oppressive feeling of worry, guilt" , "Strong feeling of anxiety/dread" , "Terrible butterflies" , "Anxious state" , "General feeling of anxiety" .)