Principal uniform worn by fighting men born on base (6,3)
I believe the answer is:
number one
'principal' is the definition.
(both can mean the top)
'by fighting men born on base' is the wordplay.
'by' means one lot of letters go next to another.
'fighting men' becomes 'umber' (I can't explain this - if you can you should give a lot more credence to this answer).
'born' becomes 'n'.
'base' becomes 'e' (Euler's number - base of natural logarithms).
'umber' after 'n' is 'number'.
'number'+'on'+'e'='NUMBER ONE'
'uniform worn' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for number one that I've seen before include "Wee" , "Informally and numerically, oneself" , "best record" , "Musical success" , "Informally the self, numerically speaking" .)