Tree: talk about one (7)
I believe the answer is:
conifer
'tree' is the definition.
(conifer is a kind of tree)
'talk about one' is the wordplay.
I cannot really understand how this works, but
'about' could be 'on' (a book about x is on x) and 'on' is found in the answer.
'one' could be 'er' (I've seen this in other clues) and 'er' is present in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for conifer that I've seen before include "Evergreen (generally)" , "Tree such as pine, yew etc" , "E.g. yew, pine or fir" , "Fir or larch, for example" , "Eg, pine, fir or cypress" .)