Estoril's piece of high ground (3)
I believe the answer is:
tor
'high ground' is the definition.
'estoril's piece' is the wordplay.
'piece' indicates a hidden word.
'TOR' can be found hidden inside 'estoril'.
'of' acts as a link.
(Other definitions for tor that I've seen before include "Rot on a hill in Devon" , "High, rocky, bare hill" , "Pillar of rock" , "rise" , "fell" .)