Stop heads of state trivialising every meeting (4)
I believe the answer is:
stem
'stop' is the definition.
(stemming is a kind of stopping)
'heads of state trivialising every meeting' is the wordplay.
'heads of' says to take the initial letters.
The first letters of 'state trivialising every meeting' is 'STEM'.
(Other definitions for stem that I've seen before include "Thwart; support" , "something growing?" , "Main upright of a ship's bow" , "Stalk - check" , "section of pipe" .)