Cosmological hypothesis second-rate, going by earth’s development (3,4,6)
I believe the answer is:
big bang theory
'cosmological hypothesis' is the definition.
I know nothing about this answer so I can't tell whether this works.
'second-rate going by earth's development' is the wordplay.
'second rate' becomes 'b'.
'development' indicates anagramming the letters.
'going'+'by'+'earth'='goingbyearth'
'goingbyearth' anagrammed gives 'igbangtheory'.
'b'+'igbangtheory'='BIG BANG THEORY'
Can you help me to learn more?
(Other definitions for big bang theory that I've seen before include "World creation thesis" , "Does this explain how it all began?" , "Idea of how we began" , "Model of how the universe began" .)