Crown service protecting millions (4)
I believe the answer is:
acme
'crown' is the definition.
(thesaurus)
'service protecting millions' is the wordplay.
'service' becomes 'ace' (ace is a kind of service**).
'protecting' indicates putting letters inside.
'millions' becomes 'm' (abbreviation).
'ace' enclosing 'm' is 'ACME'.
(Other definitions for acme that I've seen before include "Height of achievement" , "crisis in history" , "Aim of mountaineers" , "Point of perfection, apex" , "Point of maximum development" .)