Not much of an army, but Caesar had one (7)
I believe the answer is:
section
'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" .)