When flat on spine, they say you are included (6)
I believe the answer is:
supine
'they say you are included' is the definition.
The definition suggests an adverb but the answer is not.
'when flat on spine' is the wordplay.
'when flat' becomes 'u' (I can't explain this - if you can you should believe this answer much more).
'on' means one lot of letters goes inside another.
'u' placed into 'spine' is 'SUPINE'.
Can you help me to learn more?
(Other definitions for supine that I've seen before include "Prostrate" , "Lying face upward" , "Lethargic" , "offering no resistance" , "Inert; on one's back" .)