Rage press boss moderated (8)
I believe the answer is:
tempered
'moderated' is the definition.
('temper' can be a synonym of 'moderate')
'rage press boss' is the wordplay.
'rage' becomes 'temper' (temper means anger or rage).
'press boss' becomes 'ed' (I've seen this in other clues).
'temper'+'ed'='TEMPERED'
(Other definitions for tempered that I've seen before include "Made hard through heating and cooling" , "See 7" , "Neutralised, counterbalanced" , "Moderated, softened" , "Heat-treated" .)