Not susceptible (6)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
immune
'not susceptible' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for immune that I've seen before include "Cannot be infected" , "Resistant, unaffected" , "Exempt, invulnerable" , "Safe from attack" , "Resistant (to infection)" .)