Macs grouped together and left in lavatory (4)
I believe the answer is:
clan
'macs grouped together' is the definition.
(names of Scottish clans typically begin with Mac)
'left in lavatory' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' means one lot of letters goes inside another.
'lavatory' becomes 'can' (synonyms).
'l' put into 'can' is 'CLAN'.
'and' is the link.
(Other definitions for clan that I've seen before include "Group with common ancestor, like the Campbells" , "Macdonald, perhaps" , "Kinship group" , "Scottish family grouping" , "Kin group" .)