After negotiation, agreed about navy's defensive position (2,5)
I believe the answer is:
en garde
'navy's defensive position' is the definition.
I can't tell whether this defines the answer.
'after negotiation agreed about' is the wordplay.
'after' becomes 'n' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'negotiation' indicates anagramming the letters.
'about' indicates putting letters inside.
'agreed' is an anagram of 'egarde'.
'n' placed within 'egarde' is 'EN GARDE'.
Can you help me to learn more?
(Other definitions for en garde that I've seen before include "French opening position in fencing - protect yourself" , "Get ready to fence" , "Angered with opening position in fencing - be alert. (2,5)" , "The opening position in fencing - mind yourself" , "Warning cry in fencing" .)