Beginning of retirement in brilliant piece of land (4)
I believe the answer is:
acre
'piece of land' is the definition.
(I've seen this in another clue)
'beginning of retirement in brilliant' is the wordplay.
'beginning of' suggests taking the first letters.
'in' is an insertion indicator.
'brilliant' becomes 'ace' (ace can informally mean very good).
The initial letter of 'retirement' is 'r'.
'r' put inside 'ace' is 'ACRE'.
(Other definitions for acre that I've seen before include "once occupied by Crusaders" , "holy city" , "amount of land" , "Measure of land area" , "Israeli seaport" .)