Kafka's first novel cutting if not authentic (6)
I believe the answer is:
kosher
'authentic' is the definition.
(I know that kosher is a more specific form of the attribute authentic)
'kafka's first novel cutting if not' is the wordplay.
'kafka's first' becomes 'k' (1st letter of 'kafka').
'novel' becomes 'she' (novel by H Rider Haggard).
'cutting' indicates putting letters inside (some letters cut their way into a word).
'if not' becomes 'or' (both can mean 'otherwise...').
'she' put into 'or' is 'osher'.
'k'+'osher'='KOSHER'
(Other definitions for kosher that I've seen before include "Ritually clean (food)" , "over there OK?" , "Legitimate (Jewish food)" , "Edible for Jew" , "Pure (Jewish)" .)