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" .)
(Other definitions for slot that I've seen before include "Scheduled time - hierarchical position" , "Scheduled time -- aperture for coins" , "Hole for e.g. coin" , "Small opening, for a coin say" , "Space for coin or programme" .)