Practice of exhibiting all diamonds is adopted by miners (6)
I believe the answer is:
nudism
'practice' is the definition.
(I know that nudism is a type of practice)
'diamonds is adopted by miners' is the wordplay.
'diamonds' becomes 'd' (abbreviation for the card suit diamonds).
'adopted by' means one lot of letters goes inside another (I've seen this in other clues).
'miners' becomes 'NUM' (National Union of Mineworkers).
'd'+'is'='dis'
'dis' going into 'num' is 'NUDISM'.
'of exhibiting all' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for nudism that I've seen before include "Naturism" , "Practice of going without clothes" , "Mind us (anag)" , "Public exposure" , "Having nothing on" .)