Rock legend ultimately I am beside almost whole day (7)

I believe the answer is:

diamond

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'rock' is the definition.
(thesaurus)

'legend ultimately i am beside almost whole day' is the wordplay.
'ultimately' says to take the final letters.
'beside' becomes 'on'.
'almost whole day' becomes 'd' (I can't justify this - if you can you should believe this answer much more).
The last letter of 'legend' is 'd'.
'd'+'i'+'am'+'on'+'d'='DIAMOND'

Can you help me to learn more?

(Other definitions for diamond that I've seen before include "American pitch" , "Kohinoor, Hope" , "one of thirteen?" , "Precious stone of pure crystallized carbon" , "shape" .)

I've seen this clue in The Telegraph.

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