… post filled by commander (6)

I believe the answer is:

second

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'post filled by commander' is the definition.
The answer and definition can be both people as well as being singular nouns.
Maybe you can see a link between them that I don't see?

'post filled by commander' is the wordplay.
'post' becomes 'send' (synonyms).
'filled by' means one lot of letters goes inside another.
'commander' becomes 'co'.
'send' enclosing 'co' is 'SECOND'.

Can you help me to learn more?

(Other definitions for second that I've seen before include "bit" , "after one" , "Eighty six thousand and four hundredth of a day" , "Unit of time - after No 1" , "Mo" .)

I've seen this clue in The Guardian.

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