Opportunist's day at last for court (8)
I believe the answer is:
chancery
'court' is the definition.
(chancery is a kind of court)
'opportunist's day at last' is the wordplay.
'opportunist' becomes 'chancer' (I've seen this before).
'at last' suggests the final letters.
The last letter of 'day' is 'y'.
'chancer'+'y'='CHANCERY'
'for' acts as a link.
(Other definitions for chancery that I've seen before include "Division of the British courts" , "He can cry (anag.)" , "in which a wrestler may be held" , "Embassy office" , "Embassy/consulate office" .)