Advantage duke found in, say, opening of estate (4)
I believe the answer is:
edge
'advantage' is the definition.
(I've seen this before)
'duke found in say opening of estate' is the wordplay.
'duke' becomes 'd'.
'found in' indicates putting letters inside.
'say' becomes 'eg'.
'opening of' says to take the initial letters.
The first letter of 'estate' is 'e'.
'd' put into 'eg' is 'edg'.
'edg'+'e'='EDGE'
(Other definitions for edge that I've seen before include "Bind" , "Border, fringe" , "Sidle, lip" , "Border; intense quality" , "Move carefully; limit" .)