Case of diamonds? (4)
I believe the answer is:
suit
I believe this clue is a double definition.
'case' is the first definition.
(I know that case can be written as suit)
'diamonds?' is the second definition.
(diamonds is a playing card suit)
'of' is the link.
(Other definitions for suit that I've seen before include "Jacket and trouser set" , "Legal action - courtship" , "diamonds, perhaps" , "Legal action - hearts, perhaps" , "Quarter of a full deck" .)