Alternative plant knowledge some think is challenging (6,7)
I believe the answer is:
rocket science
'some think is challenging' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.
'alternative plant knowledge' is the wordplay.
'alternative plant' becomes 'rocket' (rocket is a kind of plant. I am not sure about the 'alternative' bit.).
'knowledge' becomes 'science' (science is a kind of knowledge).
'rocket'+'science'='ROCKET SCIENCE'
Can you help me to learn more?
(Other definitions for rocket science that I've seen before include "Something said to require great intelligence (usually referred to in the negative)" , "Great technical skill" , "Proverbial activity requiring greater intelligence?" , "Scenic eco-trek (anag) -- complex subject for boffins?" , "Something very difficult to understand (involving gravity?)" .)