It's part of the plot in Isaac's novel (6)
I believe the answer is:
cassia
'it's part of the plot' is the definition.
I can't tell whether this defines the answer.
'isaac's novel' is the wordplay.
'novel' indicates an anagram (letters in a new or novel order).
'isaacs' is an anagram of 'CASSIA'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for cassia that I've seen before include "Coarse bark" , "Medicinal shrub with an aromatic bark" , "Senna-producing tree" , "Tree providing senna" , "Chinese tree with aromatic bark - is a sac (anag)" .)