Eastern European’s singular toilet (4)
I believe the answer is:
slav
'eastern european's' is the definition.
(the Slavs are a European people)
'singular toilet' is the wordplay.
'singular' becomes 's' (abbreviation in grammar).
'toilet' becomes 'lav' (lavatory).
's'+'lav'='SLAV'
(Other definitions for slav that I've seen before include "One of the European people" , "Eastern European" , "A Russian, say" , "East European - Asian Russian" , "Native of eastern Europe, perhaps" .)