Letter from those who were kept in reserve (5)

I believe the answer is:

theta

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'letter from those who were kept in reserve' is the definition.
'theta' can be an answer for 'letter' (Greek letter). I am not certain of the remainder of the definition.

'letter from those who' is the wordplay.
'letter' becomes 'm'.
'from' suggests deleting specific letters.
'those' becomes 'them'.
'who' becomes 'ta' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'them' with 'm' taken away is 'the'.
'the'+'ta'='THETA'

Can you help me to learn more?

(Other definitions for theta that I've seen before include "Letter before iota" , "letter from Athens" , "Letter from Greece" , "Eighth letter of Greek alphabet" , "character in Marathon" .)

I've seen this clue in The Independent.

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