Usefulness of a public toilet (11)
I believe the answer is:
convenience
This clue is a double definition.
'usefulness' is the first definition.
(thesaurus)
'a public toilet' is the second definition.
('convenience' can be a synonym of 'public toilet')
'of' acts as a link.
(Other definitions for convenience that I've seen before include "Facility" , "gentlemen" , "Helpful device, toilet" , "Useful device; freedom from effort" , "A comfort" .)