Potty is available for president (7)
I believe the answer is:
madison
'for president' is the definition.
(James Madison, fourth president of the USA)
'potty is available' is the wordplay.
'potty' becomes 'mad' (potty can informally mean mad).
'available' becomes 'on' (I've seen this in other clues).
'mad'+'is'+'on'='MADISON'
(Other definitions for madison that I've seen before include "James, fourth US president" , "State capital of Wisconsin" , "NY avenue associated with the advertising industry" , "-- Avenue, New York's advertising district" , "United States president -- Wisconsin's capital" .)