Lad hooligan knocked over (3)
I believe the answer is:
boy
'lad' is the definition.
(thesaurus)
'hooligan knocked over' is the wordplay.
'hooligan' becomes 'yob' ('yob' can be a synonym of 'hooligan').
'knocked over' is a reversal indicator.
'yob' reversed gives 'BOY'.
(Other definitions for boy that I've seen before include "Son" , "Youth" , "Youthful male" , "Young male" , "Male child" .)