Fit in (6)
'fit in' is the definition.
(to belong somewhere is to fit in there)
This is the entire clue.
(Other definitions for belong that I've seen before include "Be the property of" , "Have a proper place (in)" , "Be correctly assigned" , "Appertain" , "find a place?" .)
(Other definitions for assimilate that I've seen before include "thoroughly understood" , "Absorb and digest (food)" , "Take in and understand" , "Become part of the prevailing culture" , "pick up" .)