Family left in lavatory (4)
I believe the answer is:
clan
'family' is the definition.
(thesaurus)
'left in lavatory' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' means one lot of letters goes inside another.
'lavatory' becomes 'can' ('can' can be a synonym of 'lavatory').
'l' placed inside 'can' is 'CLAN'.
(Other definitions for clan that I've seen before include "Group of families with common ancestor" , "People related by blood and marriage" , "Group of related families" , "Kin group" , "Scots family grouping" .)