Use of a revolving toilet with one in (5)
I believe the answer is:
avail
'use' is the definition.
(availing is a kind of using)
'a revolving toilet with one in' is the wordplay.
'revolving' says the letters should be written backwards.
'toilet' becomes 'lav' (lavatory).
'with' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral).
'in' means one lot of letters goes inside another.
'lav' reversed gives 'val'.
'a'+'val'='aval'
'aval' enclosing 'i' is 'AVAIL'.
'of' acts as a link.
(Other definitions for avail that I've seen before include "Serve a purpose" , "Be of value to" , "Be useful" , "Make use, take advantage" , "Be of service to" .)