Heads of the assembly cautiously try diplomacy (4)
I believe the answer is:
tact
'diplomacy' is the definition.
(tact can mean diplomacy or sensitivity)
'heads of the assembly cautiously try' is the wordplay.
'heads of' says to take the initial letters.
The first letters of 'the assembly cautiously try' is 'TACT'.
(Other definitions for tact that I've seen before include "Sensitivity, diplomacy" , "Diplomatic sensitivity" , "Delicacy of feeling, skill in saying the right thing" , "Sensitivity (in dealing with others)" , "Consideration in dealing with others" .)