Call for quiet, keeping debate calm (6)
I believe the answer is:
smooth
'calm' is the definition.
('smooth' can be similar in meaning to 'calm')
'call for quiet keeping debate' is the wordplay.
'call for quiet' becomes 'sh'.
'keeping' indicates putting letters inside.
'debate' becomes 'moot' ('moot' can be a synonym of 'debate').
'sh' going around 'moot' is 'SMOOTH'.
(Other definitions for smooth that I've seen before include "Calm" , "Even-surfaced" , "Not uneven or rough" , "Free from roughness, bumps etc." , "Still" .)