I usurp, so false as a Pretender (8)
I believe the answer is:
spurious
'as a pretender' is the definition.
(I know this)
'i usurp so false' is the wordplay.
'false' indicates an anagram.
'i'+'usurp'+'so'='iusurpso'
'iusurpso' anagrammed gives 'SPURIOUS'.
(Other definitions for spurious that I've seen before include "Not genuine or authentic" , "Inauthentic" , "deceitful!" , "Faked" , "False, fake" .)