Not liable (6)
'not liable' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for exempt that I've seen before include "Permitted to be unaffected" , "Excused - let off" , "Freed from obligation, tax etc" , "Duty-free" , "Free from, eg, tax" .)
(Other definitions for immune that I've seen before include "Not liable to, protected from" , "Cannot be infected" , "Inoculated or exempt" , "Secure against, infection say" , "Resistant (to infection)" .)