Bank on lavatory being vacant (4)
I believe the answer is:
rely
'bank' is the definition.
(to bank on is to rely on)
'on lavatory being vacant' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'being vacant' means to remove the middle letters (a vacant or emptied out form of the word).
'lavatory' with its middle taken out is 'ly'.
're'+'ly'='RELY'
(Other definitions for rely that I've seen before include "Have confidence or faith in" , "Be sure" , "Run and see" , "Depend, bank" , "Bank (on someone)" .)