Lav (3)
I believe the answer is:
loo
'lav' is the definition.
(I know that loo is a type of lav)
This is the entire clue.
(Other definitions for loo that I've seen before include "Lavatory - old card game" , "In which one goes" , "Card game; the 'smallest room'" , "Toilet (informal)" , "The little room - card game" .)