Resistant to infection (6)
I believe the answer is:
immune
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">resistant to infection</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)