Authentication is tearing us apart (9)
I believe the answer is:
signature
'authentication' is the definition.
(thesaurus)
'tearing us apart' is the wordplay.
'apart' indicates an anagram (the word is broken apart).
'tearing'+'us'='tearingus'
'tearingus' is an anagram of 'SIGNATURE'.
'is' acts as a link.
(Other definitions for signature that I've seen before include "Distinctive mark" , "Mark of individuality" , "Personal mark" , "Handwritten name" , "Distinctive way of writing name" .)