Where e.g. Arabs may be penning story is not in doubt (13)
I believe the answer is:
incontestable
'where e g arabs' is the definition.
The definition suggests a singular noun but the answer is not.
'be penning story is not in doubt' is the wordplay.
'be penning' is an insertion indicator.
'story' becomes 'conte' (I've seen this before).
'is not in doubt' becomes 'instable' (I can't explain this - if you can you should believe this answer much more).
'conte' inserted into 'instable' is 'INCONTESTABLE'.
'may' acts as a link.
Can you help me to learn more?
(Other definitions for incontestable that I've seen before include "Beyond dispute" , "sure" , "Without doubt" .)