Affected by bacteria round top of commode? One's not convinced (7)
I believe the answer is:
sceptic
'one's not convinced' is the definition.
'affected by bacteria round top of commode?' is the wordplay.
'affected by bacteria' becomes 'septic'.
'round' is an insertion indicator.
'top of' says to take the initial letters.
The first letter of 'commode' is 'c'.
'septic' enclosing 'c' is 'SCEPTIC'.
(Other definitions for sceptic that I've seen before include "Thomas for one" , "Person who tends to disbelieve" , "A doubting Thomas" , "Person who doubts the truth" , "Disbeliever" .)