Talk together about one tree (7)
I believe the answer is:
conifer
'tree' is the definition.
(conifer is a kind of tree)
'talk together about one' is the wordplay.
'talk together' becomes 'confer' (I've seen this before).
'about' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'confer' enclosing 'i' is 'CONIFER'.
(Other definitions for conifer that I've seen before include "In force (anag.)" , "Eg, pine, fir or cypress" , "The tree keeps its leaves" , "Tree such as pine, yew etc" , "one branching out" .)