Have a discussion about one tree (7)
I believe the answer is:
conifer
'tree' is the definition.
(conifer is a kind of tree)
'have a discussion about one' is the wordplay.
I cannot quite see how this works, but
'about' could be 'on' (as in a book 'on' a certain topic) and 'on' is located in the answer.
'one' could be 'i' (Roman numeral) and 'i' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for conifer that I've seen before include "Fir or pine, for example" , "The tree doesn't lose its leaves" , "Tree such as pine, yew etc" , "one branching out" , "Evergreen tree" .)