Ornate rosebush inlaid with first of many diamonds (9)

I believe the answer is:

rhombuses

'diamonds' is the definition.
(I've seen this before)

'ornate rosebush inlaid with first of many' is the wordplay.
'ornate' is an anagram indicator (an unusual or complex form of the letters).
'inlaid with' indicates putting letters inside.
'first of many' becomes 'm' (1st letter of 'many').
'rosebush' anagrammed gives 'rhobuses'.
'rhobuses' enclosing 'm' is 'RHOMBUSES'.

(Another definition for rhombuses that I've seen is " Diamonds".)

I've seen this clue in The Times.

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