Belligerent mother's on trial when leader demoted (7)
I believe the answer is:
martial
'belligerent' is the definition.
(I've seen this before)
'mother's on trial when leader demoted' is the wordplay.
I cannot quite see how this works, but
an anagram of 'trial' is 'rtial' which is present in the answer.
'mother' could be 'ma' (ma can mean mother) and 'ma' is found within the answer.
This accounts for all the letters.
This explanation may well be incorrect...
(Other definitions for martial that I've seen before include "Specialist in epigrams" , "Relating to the military" , "These arts concern fighting sports" , "Warlike; Roman poet" , "Of war or the armed forces" .)