Vertebrae keeping you reportedly horizontal (6)

I believe the answer is:

supine

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'reportedly horizontal' is the definition.
'supine' can be an answer for 'horizontal' (thesaurus). I am not sure about the 'reportedly' bit.

'vertebrae keeping you' is the wordplay.
'vertebrae' becomes 'spine'.
'keeping' is an insertion indicator.
'you' becomes 'u' (I've seen this in other clues).
'spine' going around 'u' is 'SUPINE'.

Can you help me to learn more?

(Other definitions for supine that I've seen before include "Slow in acting" , "Unresisting" , "Prostrate" , "Laid-back" , "offering no resistance" .)

I've seen this clue in The Telegraph.

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