I clean out boils to protect against infection (9)
I believe the answer is:
inoculate
'protect against infection' is the definition.
(inoculation protects against infection)
'i clean out boils' is the wordplay.
'boils' indicates an anagram (I've seen 'boiled' mean this (letters cooked or boiled into a new form)).
'clean'+'out'='cleanout'
'cleanout' with letters rearranged gives 'noculate'.
'i'+'noculate'='INOCULATE'
'to' acts as a link.
(Other definitions for inoculate that I've seen before include "Treat with a vaccine or serum" , "Induce immunity" , "to prevent infection" , "Immunise, vaccinate" , "Graft" .)