Accountants beside old tree (5)
I believe the answer is:
cacao
'tree' is the definition.
(cacao tree is a kind of tree)
'accountants beside old' is the wordplay.
'accountants' becomes 'caca'.
'beside' says to put letters next to each other (I've seen this in other clues).
'old' becomes 'o' (abbreviation).
'caca'+'o'='CACAO'
(Other definitions for cacao that I've seen before include "Plant" , "Tree from which chocolate is made" , "Tropical tree, the source of chocolate" , "Tree for cocoa bean" , "Tree with seeds used to make chocolate" .)