Conservative on mission for mastery (8)
I believe the answer is:
conquest
'mastery' is the definition.
(thesaurus)
'conservative on mission' is the wordplay.
'conservative' becomes 'con' (abbreviation for Conservative).
'on' says to put letters next to each other.
'mission' becomes 'quest' (a quest is a mission or search).
'con'+'quest'='CONQUEST'
'for' acts as a link.
(Other definitions for conquest that I've seen before include "Catch" , "Victory by force" , "hostile takeover" , "occupied territory" , "Taking of another's territory" .)