Militant Clair upset to receive notice (7)
I believe the answer is:
radical
'militant' is the definition.
(I've seen this before)
'clair upset to receive notice' is the wordplay.
'upset' indicates anagramming the letters.
'to receive' indicates putting letters inside.
'notice' becomes 'ad' (both can mean an advertisement).
'clair' is an anagram of 'rical'.
'rical' placed around 'ad' is 'RADICAL'.
(Other definitions for radical that I've seen before include "Going to the roots, fundamental as change may be" , "Complete" , "Going to the root" , "One with rooted" , "Total" .)