A mad activist, displaying primitive behaviour (9)
I believe the answer is:
atavistic
'displaying primitive behaviour' is the definition.
'a mad activist' is the wordplay.
'mad' indicates an anagram.
'activist' with letters rearranged gives 'tavistic'.
'a'+'tavistic'='ATAVISTIC'
(Other definitions for atavistic that I've seen before include "Old-fashioned" , "Involving reversion to an earlier type" , "Reverting to an ancestral form" , "looking back to earlier days" , "Like a remote ancestor or throwback" .)