The post of an officer ranking between major and brigadier (9)

'the post' is the definition.
Both the definition and answer are singular nouns.
Perhaps you can see an association between them that I don't see?

'an officer ranking between major and brigadier' is the wordplay.
I cannot quite understand how this works, but
'an' could be 'one' and 'one' is found within the answer.
'officer' could be 'col' (short for colonel) and 'col' is present in the answer.
This may be the basis of the clue (or it may be nonsense).

'of' acts as a link.

Can you help me to learn more?