Established tree on top of mound (4)
I believe the answer is:
firm
'established' is the definition.
('firm' can be a synonym of 'established')
'tree on top of mound' is the wordplay.
'tree' becomes 'fir' (fir is a kind of tree**).
'on top of' says to take the initial letters (I've seen 'on top' mean this).
The initial letter of 'mound' is 'm'.
'fir'+'m'='FIRM'
(Other definitions for firm that I've seen before include "Resistant to pressure" , "Not soft or yielding to pressure" , "Steady kind of business" , "Company - solid" , "Secure - business partnership" .)