Depend on vacated lavatory (4)
I believe the answer is:
rely
'depend' is the definition.
(thesaurus)
'on vacated lavatory' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'vacated' suggests removing the centre.
'lavatory' with its middle removed is 'ly'.
're'+'ly'='RELY'
(Other definitions for rely that I've seen before include "Bank (on someone)" , "Run and see" , "Trust, depend" , "Have confidence or faith in" , "Old rally" .)