Case of diamonds, say (4)
I believe the answer is:
suit
I believe this is a double definition.
'case' is the first definition.
(synonyms)
'diamonds say' is the second definition.
(diamonds are a playing card suit)
'of' is the link.
(Other definitions for suit that I've seen before include "It constitutes one quarter of a deck" , "Formal wear (for court action?)" , "Administrative official" , "Legal action - courtship" , "clubs, for example" .)