Rhea, perhaps under three feet, captive in the US (8)

I believe the answer is:

yardbird

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'captive in the us' is the definition.
The definition suggests a singular noun which matches the answer.

'rhea perhaps under three feet' is the wordplay.
'rhea perhaps' becomes 'bird' (rhea is a kind of bird).
'under' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'three feet' becomes 'yard' (I've seen this before).
'bird' after 'yard' is 'YARDBIRD'.

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I've seen this clue in The Times.

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