In amphitheatre, a sure spot for prizes (9)
I believe the answer is:
treasures
'for prizes' is the definition.
('treasure' can be a synonym of 'prize')
'in amphitheatre a sure spot' is the wordplay.
'in' says the answer is hidden in the clue.
'TREASURES' is hidden within 'amphitheatre a sure spot'.
(Other definitions for treasures that I've seen before include "lots of valuables" , "Values" , "Holds dear, prizes" .)