Friend at outset mentions tree (4)
I believe the answer is:
palm
'tree' is the definition.
(palm tree is a kind of tree)
'friend at outset mentions' is the wordplay.
'friend' becomes 'pal' (pal is a kind of friend).
'at outset' says to take the initial letters.
The first letter of 'mentions' is 'm'.
'pal'+'m'='PALM'
(Other definitions for palm that I've seen before include "Sign of victory" , "Tree with no branches and large leaves" , "Underside of hand" , "Conceal in the hand" , "Use hand" .)