Sunday? That's right, man (7)

I believe the answer is:

soldier

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'man' is the definition.
(I know that soldier is a type of enlisted person)

'sunday? that's right' is the wordplay.
'sunday?' becomes 's'.
'that's' becomes 'oldie' (I can't justify this - if you can you should give a lot more credence to this answer).
'right' becomes 'r' (common abbreviation).
's'+'oldie'+'r'='SOLDIER'

Can you help me to learn more?

(Other definitions for soldier that I've seen before include "One of man's 22 20 [SEVEN AGES]" , "Tommy Atkins or GI Joe?" , "20 down man" , "One enlisted" , "Warrior" .)

I've seen this clue in The Guardian.

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