To guard against infection I clean out potty (9)
I believe the answer is:
inoculate
'to guard against infection' is the definition.
'i clean out potty' is the wordplay.
'potty' indicates anagramming the letters.
'clean'+'out'='cleanout'
'cleanout' anagrammed gives 'noculate'.
'i'+'noculate'='INOCULATE'
(Other definitions for inoculate that I've seen before include "Introduce an idea or attitude into the mind of: immunise" , "Give a jab" , "Imbue" , "give someone the needle" , "Immunise, vaccinate" .)