Herbivore long ago sorted out grasses, mostly (9)

I believe the answer is:

stegosaur

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'herbivore long ago' is the definition.
I can't tell whether this definition defines the answer.

'sorted out grasses mostly' is the wordplay.
'sorted' indicates anagramming the letters.
'mostly' means to remove the last letter.
'out'+'grasses'='outgrasses'
'outgrasses' with its last letter removed is 'outgrasse'.
'outgrasse' is an anagram of 'STEGOSAUR'.

Can you help me to learn more?

(Other definitions for stegosaur that I've seen before include "Armoured fossil creature" , "Jurassic creature" , "old dinosaur" , "prehistoric creature" , "Old beast" .)

I've seen this clue in The Guardian.

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