Resistant antibodies run amok (13)
I believe the answer is:
insubordinate
'resistant' is the definition.
('insubordinate' can be a synonym of 'resistant')
'antibodies run amok' is the wordplay.
'amok' is an anagram indicator.
'antibodies'+'run'='antibodiesrun'
'antibodiesrun' with letters rearranged gives 'INSUBORDINATE'.
(Other definitions for insubordinate that I've seen before include "Insolent" , "Not submissive to authority" , "Badly behaved" , "Recalcitrant" , "Cheeky" .)