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The Alamo, USA, most of all a vital relay station (8)

Ross

I think it could be either:

thalamus

narrates
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'vital relay station' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.

'the alamo usa most of all a' is the wordplay.
I cannot really understand how this works, but
'usa' could be 'us' (synonyms) and 'us' is present in the answer.
'a' is found in the answer.
This may be the basis of clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for thalamus that I've seen before include "bit of brain" , "a bit of brains" , "Sensory part of brain; a lush mat (anag.)" , "Grey matter at base of brain" .)

I've seen this clue in The Guardian.
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