Promoted piece of exquisitely graceful diamond-based design (6)
I believe the answer is:
argyle
'based design' is the definition.
'argyle' can be an answer for 'design' (I have seen 'Woolly design' mean 'argyle' so perhaps 'design' could also mean 'argyle'). I am unsure of the 'based' bit.
'promoted piece of exquisitely graceful diamond' is the wordplay.
'promoted' shows that the letters should be reversed in order (going up, in a down clue).
'piece of' says the answer is hidden in the clue.
'elygra' is hidden in the letters of 'exquisitely graceful diamond'.
'elygra' written backwards gives 'ARGYLE'.
Can you help me to learn more?
(Other definitions for argyle that I've seen before include "Lozenge-patterned sock" , "Of diamond - patterned material" , "Knitted or woven with diamond-shaped pattern" , "Old Scottish county; part name of a W Country football team" , "Diamond-shaped pattern for woollens" .)