Top class haven for millions — it can't be bettered (4)
I believe the answer is:
acme
'top' is the definition.
(synonyms)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for acme that I've seen before include "Height of excellence" , "crisis in history" , "'Highest point, apex (4)'" , "Highest point (of something)" , "Highest level or degree attainable" .)