Prohibits discussion about base in plant stem (9)
I believe the answer is:
beanstalk
'plant stem' is the definition.
'beanstalk' can be an answer for 'stem' (beanstalk is a kind of stem). I'm unsure of the 'plant' bit.
'prohibits discussion about base in' is the wordplay.
'prohibits' becomes 'bans' (banning is a kind of prohibiting).
'discussion' becomes 'talk' (talk is a kind of discussion).
'about base' becomes 'e' (Euler's number - base of natural logarithms. I am not sure about the 'about' bit.).
'in' means one lot of letters goes inside another.
'bans'+'talk'='banstalk'
'banstalk' enclosing 'e' is 'BEANSTALK'.
Can you help me to learn more?
(Other definitions for beanstalk that I've seen before include "Entry to giant's home?" , "Jack cut it down" , "See 33" , "way up for fortune-hunter" , "Leguminous stem that Jack climbed" .)