Crossword Genius

Not much of an army, but Caesar had one (7)

Ross

I believe the answer is:

section

I'm a little stuck... Click here to teach me more about this clue!

'one' is the definition.
The answer and definition can be both quantities as well as being singular nouns.
Maybe you can see an association between them that I don't see?

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for section that I've seen before include "Element" , "Divide; part" , "Notices (anag)" , "Division, portion" , "Department, division" .)

I've seen this clue in the Evening Standard.
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