Resistant to infection (6)
I believe the answer is:
immune
'resistant to infection' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for immune that I've seen before include "Resistant (to infection)" , "Secure against, infection say" , "Not susceptible to, say after inoculation" , "Exempt or protected" , "Resistant, unaffected" .)