May well vomit, oddly tired at last of marrow (7)

I believe the answer is:

myeloid

'of marrow' is the definition.
(I know this)

'may well vomit oddly tired at last' is the wordplay.
'oddly' indicates alternate letters (letters in odd-numbered positions).
'at last' indicates one should take the final letters.
The last letter of 'tired' is 'd'.
'may'+'well'+'vomit'='maywellvomit'
The alternating letters of 'maywellvomit' are 'myeloi'.
'myeloi'+'d'='MYELOID'

(Another definition for myeloid that I've seen is " in spinal cord".)

I've seen this clue in The Guardian.

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