Friend on top of mulberry tree (4)
I believe the answer is:
palm
'tree' is the definition.
(I know that palm is a type of tree)
'friend on top of mulberry' is the wordplay.
'friend' becomes 'pal' (pal can mean a friend).
'on top of' suggests taking the first letters.
The first letter of 'mulberry' is 'm'.
'pal'+'m'='PALM'
(Other definitions for palm that I've seen before include "Secretly snatch" , "Tropical tree or part of hand" , "Obscure" , "Symbol of victory" , "that's inspected by a fortune-teller" .)