Does up building that's not authentic (6)
I believe the answer is:
pseudo
'not authentic' is the definition.
'does up building' is the wordplay.
'building' is an anagram indicator.
'does'+'up'='doesup'
'doesup' anagrammed gives 'PSEUDO'.
'that's' is the link.
(Other definitions for pseudo that I've seen before include "Prefix for fake" , "Insincere person" , "Fake, mock, false" , "'Fake, sham (6)'" , "Counterfeit" .)