Bone, green then black in time (8)

I believe the answer is:

vertebra

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'bone' is the definition.
(I know that vertebra is a type of bone)

'green then black in time' is the wordplay.
'green then' becomes 'vert' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'black' becomes 'b' (as in pencil classifications).
'in' is an insertion indicator.
'time' becomes 'era' (era is a kind of time).
'b' going within 'era' is 'ebra'.
'vert'+'ebra'='VERTEBRA'

Can you help me to learn more?

(Other definitions for vertebra that I've seen before include "Segment of the spine" , "Disconcerted" , "spinal unit" , "bit of backbone" , "Backbone segment" .)

I've seen this clue in The Telegraph.

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