Solitary working lavatory's vacant (4)
I believe the answer is:
only
'solitary' is the definition.
(I know that solitary can be written as only)
'working lavatory's vacant' is the wordplay.
'working' becomes 'on' (on can mean working or activated).
'vacant' says to hollow out the word (remove centre letters) (a vacant or emptied out form of the word).
'lavatory' with its centre removed is 'ly'.
'on'+'ly'='ONLY'
(Other definitions for only that I've seen before include "Single, solitary" , "Existing alone with no other, perhaps a child" , "Alone; solitary" , "Solely; except that" , "And nothing more; sole" .)