Greatest achievement of expert enthralling millions (4)
I believe the answer is:
acme
'greatest achievement' is the definition.
(acme can mean a peak or summit)
'expert enthralling millions' is the wordplay.
'expert' becomes 'ace' (ace can informally mean very good).
'enthralling' indicates putting letters inside.
'millions' becomes 'm' (abbreviation).
'ace' going around 'm' is 'ACME'.
'of' acts as a link.
(Other definitions for acme that I've seen before include "high peak" , "Culmination" , "crisis in history" , "Point of maximum development" , "High point of excellence" .)