Very hard substance – card suit (7)
I believe the answer is:
diamond
This clue is a double definition.
'very hard substance' is the first definition.
(I've seen this before)
'card suit' is the second definition.
(I know that diamond is a type of card suit)
(Other definitions for diamond that I've seen before include "Sixtieth" , "Very hard substance" , "Precious stone for mad Odin" , "A suit; a gem" , "Carbon gem" .)