Presumed substantial line appears in E Nesbit novel (10)
I believe the answer is:
ostensible
'presumed' is the definition.
(I know that presumed can be written as ostensible)
'in e nesbit novel' is the wordplay.
'in' indicates putting letters inside.
'e' becomes 'osle' (I can't explain this - if you can you should give a lot more credence to this answer).
'novel' is an anagram indicator (letters in a new or novel order).
'nesbit' with letters rearranged gives 'tensib'.
'osle' placed around 'tensib' is 'OSTENSIBLE'.
'substantial line appears' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for ostensible that I've seen before include "Apparently true" , "Supposed" , "Apparent" , "Seeming" , "Professed" .)