Label as "an adherent" (7)
I believe the answer is:
sticker
This clue is a double definition.
'label' is the first definition.
(synonyms)
'an adherent' is the second definition.
(thesaurus)
'as' is the link.
(Other definitions for sticker that I've seen before include "Determined person" , "Person with staying power" , "he soldiers on!" , "Gummed label" , "problem" .)