Gangster Robin left with hesitation (7)
I believe the answer is:
hoodlum
'gangster' is the definition.
(I've seen this before)
'robin left with hesitation' is the wordplay.
'robin' becomes 'hood' (I've seen this in other clues).
'left' becomes 'l' (common abbreviation).
'with' says to put letters next to each other.
'hesitation' becomes 'um' (sound made when hesitating).
'hood'+'l'+'um'='HOODLUM'
(Other definitions for hoodlum that I've seen before include "Punk" , "Young thug or criminal" , "Hooligan" , "Street bully" , "Aggressive young criminal" .)