Book one group including bass for a stellar opening? (3,4)
I believe the answer is:
big bang
'a stellar opening?' is the definition.
(origin of the universe)
'book one group including bass' is the wordplay.
'book' becomes 'b' (abbreviation for book).
'one' becomes 'I' (Roman numeral).
'group' becomes 'gang' (a gang is a group of people).
'including' indicates putting letters inside.
'bass' becomes 'b' (musical abbreviation).
'gang' going around 'b' is 'gbang'.
'b'+'i'+'gbang'='BIG BANG'
'for' is the link.
(Other definitions for big bang that I've seen before include "Moment one" , "beginning to matter?" , "First ever explosion?" , "Beginning of the universe according to theory" , "Universal beginning" .)