Group showing concern about diamonds (5)
I believe the answer is:
cadre
'group showing' is the definition.
'cadre' can be an answer for 'group' (I've seen this before). I am unsure of the 'showing' bit.
'concern about diamonds' is the wordplay.
'concern' becomes 'care' (synonyms).
'about' is an insertion indicator.
'diamonds' becomes 'd' (abbreviation in card games).
'care' enclosing 'd' is 'CADRE'.
Can you help me to learn more?
(Other definitions for cadre that I've seen before include "Small (military) unit capable of expansion when necessary" , "activist group" , "Raced around with group" , "team" , "Raced around with basic military unit" .)