Spineless, returning American tree (6)

I believe the answer is:

supine

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'spineless' is the definition.
Both the answer and definition are adjectives. Maybe you can see a link between them that I can't see?

'returning american tree' is the wordplay.
'returning' is a reversal indicator.
'american' becomes 'US' (abbreviation for United States).
'tree' becomes 'pine' (pine tree is a kind of tree).
'us' backwards is 'su'.
'su'+'pine'='SUPINE'

Can you help me to learn more?

(Other definitions for supine that I've seen before include "Lying face upwards, indolent" , "Prostrate" , "Recumbent" , "looking up" , "offering no resistance" .)

I've seen this clue in The Daily Express.

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