Mutual manoeuvring about period not concluding? This fixes an endpoint (9)
I believe the answer is:
ultimatum
'this fixes an endpoint' is the definition.
'mutual manoeuvring about period not concluding?' is the wordplay.
'manoeuvring' indicates anagramming the letters.
'about' is an insertion indicator.
'period' becomes 'time' (time is a kind of period).
'not concluding?' means to remove the last letter.
'mutual' anagrammed gives 'ulatum'.
'time' with its last letter taken off is 'tim'.
'ulatum' enclosing 'tim' is 'ULTIMATUM'.
(Other definitions for ultimatum that I've seen before include "Threat" , "Final (peremptory) demand" , "Uncompromising demand" , "Final statement of actions required of another" , "Final warning" .)