Tim in mutual restructured final demand (9)
I believe the answer is:
ultimatum
'final demand' is the definition.
(I've seen this before)
'tim in mutual restructured' is the wordplay.
'in' indicates putting letters inside.
'restructured' indicates anagramming the letters (I've seen 'restructuring' mean this).
'mutual' anagrammed gives 'ulatum'.
'tim' going within 'ulatum' is 'ULTIMATUM'.
(Other definitions for ultimatum that I've seen before include "Final (peremptory) demand" , "Threat" , "Final offer" , "Final statement of actions required of another" , "Final proposition or demand" .)