Hidden diamonds in country (5)
I believe the answer is:
perdu
'hidden' is the definition.
(I've seen this in another clue)
'diamonds in country' is the wordplay.
'diamonds' becomes 'd' (abbreviation in bridge say).
'in' is an insertion indicator.
'country' becomes 'peru' (Peru is an example).
'd' put inside 'peru' is 'PERDU'.
(Other definitions for perdu that I've seen before include "in hiding" , "lost to sight" , "secret" , "Concealed; lie in ambush" , "Concealed; reckless" .)