Serving man sides with gangster in early years (8)
I believe the answer is:
girlhood
'early years' is the definition.
(young years of a girl)
'serving man sides with gangster' is the wordplay.
'serving man' becomes 'gi' (GI can mean a soldier serving in the US army).
'sides' becomes 'RL' (abbreviations for left and right).
'with' is a charade indicator (letters next to each other).
'gangster' becomes 'hood' (I've seen this before).
'gi'+'rl'+'hood'='GIRLHOOD'
'in' acts as a link.
(Another definition for girlhood that I've seen is " State of being a female child".)